I recently had a similar situation with water in the belly of our fiver. Without going into all the detail let me just say ours was not a plumbing leak OR a leak from rain BUT a water intrusion from DRIVING in the rain.
If you have pulled the rig in the rain, I suggest you check the integrity of the underbelly fabric and any areas underneath where water could get in from driving in the rain.
I noticed this in July and it took me till last week to rule out fresh water plumbing leak, black tank flush, roof/caulking leaks from rain storms, etc. I knew it was not grey or black tank leaks due to odor - the only odor was a wet insulation smell.
I drove in a torrential rain storm on two occasions back in July. I have concluded that resulted in the water in an area of the underbelly.
It took me a while and several trips to go through the process of elimination but I'm comfortable with my deduction. The underbelly has remained bone dry through about 12 nights of use, much FL rain, and black tank flush use. The only thing I have not done is drive in the rain.
Good luck, Phillip