The RDR1 mod skips the search animation, causing scripts to think that the path to the corpse is blocked. This allows you to immediately add loot to yourself.
Go to the "content/release64/scripting/gringo/commonscripts" folder (there should be a lot of WSC/Script Resource files in there).
Move the files from this mod into the program window, and let MagicRDR replace them (if MagicRDR prompts you to import instead of replace, close it and try again).
Select File > Save > Current.
For Undead Nightmare, use the same method, but place the "Instant Loot - Undead Nightmare" files in the "content/release64/dlc/zombiepack/gringos/scripts" folder.