Originally Posted by Mixter
Is it supposed to have a cap in there? I don't see that this filler door is weather sealed. Is it supposed to be?
Fresh tank needs to be vented, but no water should run into the tank unless you put it there; rainwater runoff would be considered a source of contamination, since there's no telling how much bird and bug waste, road dust, or whatever, will end up in your tank from the runoff.
So whether the trailer came with a cap inside the fill port or not, you should put one now, just to make sure that your clean water stays clean.