Tic,
I would disconnect the battery + feed from both banks, with them fully charged. Usually a good idea to tie wrap them or tape them so they cannot find their way back to the posts.
This is just a drain trap item, when we leave the house here in the south, for the summer with home air conditioning enabled, the locals had some tricks to stop drain trap evaporation. They add just a bit of mineral oil in each drain which forms a thin oil layer over the top of the water in the trap, preventing evaporation and then also close the drain.
They also advised putting plastic wrap over the toilet to seal that some from evaporation loss. A reputable long standing RV dealer in your area may have some ventilation insight, not sure of the pros and cons of that one.
Later Ed