Not to rule out the batteries but there are other possible causes:
- Shower drain (or any drain) that has dried out and letting sewer gas back up into the camper. Add a couple cups of water to each drain.
- Fridge cooling unit died. Smells really bad!! Sometimes you will see a powder around the coil that indicates it is leaking. Replace cooling unit if this is it.
- Dead creature, find and dispose of, could even be in the bed of the truck if it is on the truck.
- Propane - already covered
- Hot water tank sulfated (not sure if that is a word), usually smells when you are running the hot water or dumping the tanks.
Good luck on finding it and be careful!