Get on xbox 360, make a RDR gamesave via going to multiplayer, then use google to search for a updated RDR mod tool. Download that tool, then go back to your xbox go to your storage and copy/move your multiplayer save to a usb or external save /storage device. Now disconnect the external storage device and plug it into your computer. Now look for a storage device explorer made for xbox 360 (I recommend horizion), now once horizon is open and your storage device is recognized in horizon, under the games tab in your storage device click RDR, now right-click your multiplayer save and extract it to your desktop or wherever you want it to be stored at. Now open the RDR mod tool you downloaded previously, and in most mod tools there will be a option in the tool that says open save, now just open the gamesave you just extracted, Now in the tool you will see options to make all guns gold and unlocked, all titles unlocked, all characters unlocked, max prestige/rank level 50, Choose mount option, most tools come preconfigured with modded mounts such as the war horse and zombie horse, and in some tools a exp editor and stats editor. Mod to your likings and then click save gamesave in the tool. Now once you have modded your gamesave, find your modded gamesave and drag and drop it into horizon, once the modded gamesave is in horizon click save,rehash, and resign at least 2-3 times, now click the save to device option and if it ask to overwrite click yes. Now safely eject your storage device from your computer and plug it back into your xbox 360, now go to storage, go to your external storage device and move/copy your new modded gamesave to your main harddrive on the xbox 360. Now load up rdr and go to multiplayer and if the tool was a updated tool for the last known update for rdr you should be max level with gold guns and everything unlocked, if it worked now head back to your storage on dashboard/settings, if cloud storage is not enabled enable it and copy your modded save to cloud storage so you can use that gamesave on xbox one.
If you don't want to mod the gamesave yourself, search on youtube or google or bing for a pre-modded xbox 360 RDR multiplayer gamesave, Best search (RDR Modded MP Gamesave level 50 max prestige all gold guns, zombie horse xbox 360) usually those keywords will throw you to a downloadable rdr modded gamesave for xbox 360, once you find one just download it, and usually/most of the time the modded gamesave will be packed into a .rar archive/folder, if it is that's no problem search for winrar, or go to rarlab site where you can download the tool to extract the gamesave out of the rar folder. Once you have winrar installed your gamesave rar folder will now look different than before just double click that file and in that file you will see the gamesave hover over the gamesave and right-click and extract the gamesave to your desktop or where ever you want it to go. Now once thats done go back to your xbox 360, self assuming you already have a RDR multiplayer gamesave, if not create one. Once that is done plug in a external usb/storage device, and go to your stroage, go to your HDD and move/copy your RDR multiplayer gamesave to your external device. Now unplug your external device and plug it into your computer, now open horizon if you have it downloaded, if not search up horizon xbox 360 mod tool, it will take you straight to the site download it and install it. Now open horizon and plug your external storage device into your computer, in horizon your storage device will be recognized now you should see tabs such as games/videos/title updates/ gamerpics, ect. Click games tab. Now go to rdr tab, now look for your RDR multiplayer gamesave(right click it and extract it to a folder called backup just incase something wrong happens down the line), just double click it and it will open, once thats open drag the window to the far left of the horizon tool and make sure that window doesnt close /minimize, now go back to your desktop or where the modded gamesave you downloaded is and drag and drop it into horizon, Another little window will pop up with the modded gamesave, go to YOUR gamesave and look for the words that say Profile/account id, highlight that and copy it, now go to the modded save and look for profile/account id and erase that id and paste yours there. Now go to YOUR save and look for console id, copy that, now go to the modded save and look for console id and erase that and paste your console id, now go to YOUR save and look for device id, copy that and go back to the modded save and look for device id and erase that and paste your device id. Now close your gamesave! via the small grey/red x on your gamesave window. GO to the modded gamesave that now has all of your id information and at the top of the window on the save it will say save, rehash and resign, click that option 2-3 times, now go to the rdr tab in your external storage device you should see your original gamesave right-click it and click remove/delete, yes you have to do this or you will get confused later on. Once thats done, go back to the modded gamesave window and you should see save to device, click that, once it has done that unplug the save device and plug it into your xbox 360. Now on your xbox 360 go to storage, go to your hard drive and go to the rdr tab, find your multiplayer gamesave and delete it, yes delete it because if you don't the game will auto load that gamesave first before it tries to load the modded gamesave. Now go to your external device and go to the rdr tab and you should see the modded gamesave, now copy/move it to your main hdd. Now load up rdr go to multiplayer you should now be level 50 max prestige, all gold guns, all titles, and characters and such unlocked depending on the type of modded gamesave, if all worked then go back to your storage and copy the modded rdr save to cloud storage so you can use it on xbox one. If the modded save didnt load and or said corrupted then you did something wrong and you just lost the modded save and your original gamesave, however on your computer you should still have the back up as previously stated, if something goes wrong just plug your save device into your computer, open horzion, then drag your save into horizon and click save to device then put the device back into the xbox 360 and move the save back to your normal hard drive.
If neither of those two options are what you were looking for then I just wasted my time writing all of that but I had nothing else better to do atm. If you are looking for a actual modded lobby, then you wont find one unless XBLToothPik host one from his mod menu which I believe to still be in development. Or you run into somebody using mine and glitched matrix's RDR V1-3 Iso mods, but those mods don't really have a amazing impact on other players, the best it can do for ranking up purposes for other players is find one of those areas where you have to shoot people and you get xp and sometimes the explosive rifle, but can do it very fast with infinite dead eye and infinite ammo.
However if you know how to burn modded games and play them on xbox 360 via hotswap or a flashed xbox then you can find them here feel free to download the rpf version only if you have the iso and know how to iso mod and burn games and to play burned games. The actual iso mod itself v3 is very fun to use on my behalf because I helped create it, you can run super fast there are hidden custom trains and sky rafts in various places, camps have been modded to have cannons and gatlin guns online players can't see or use but you can. Your feet leave a fire like trail/splash, the trees are random colors sometimes, aimlock is always enabled for that aim assist you just need, some animals have been textured for a zombie like theme some were poorly done by me, sorry i'm no expert lmao. You can use the singleplayer cheat menu (don't spawn coaches they freeze people every now and then by accident), you can get unlimited ammo and dead eye, spawn a horse or donkey, make yourself a public enemy with the largest bounty. Animals can now be mounted and rideable, the atmosphere has been changed, and much much more. However again you can't use these if you don't know how to iso mod and i'm not gonna explain that process, instead look on youtube how to iso mod xbox 360 games. It can be difficult if you are new to it. Regardless I hope this information helps you or sheds some light on what you are asking for.
Those lobbies are infection lobbies that we good mooders are trying to patch ourselves because it does more harm than good. You also can transfer your save via using xbox 360's cloud storage. Also stay away from those lobbies, those lobbies are overloaded servers with hundreds of thousands of invisible zombie npc's that are ruining the game, that spread from session to session making the game unplayable.I edited my save on 360 but it doesn’t carry over and I sold my 360 I was looking for one of them lobbies where everyone’s invisible and it ranks you up in Mcflares Ranch but thanks anyway