I have water in several storage bays. This is the first the coach, 1984 , has seen real rain, other than a trip to LA from Sioux Falls. Prior owner had indoor storage, so this may be a very old unknown problem. Battery compartments have 1/2 inch of water ,but some time in the past 1/2" x 3" slots were cut in the bottom for drainage. I had thought it was for added ventilation. One small storage compartment is even very moist in the top tray area. All gaskets look good. Perhaps leakage in the hinge?
If I remember correctly my '78 had an additional seal on the hinge where the 2 flat portions meet. It was a ribbon-like foam ....approx. 3/8' wide by 1/8 or 3/16" thick.
Guy, I've had that same issue in one, or two of my bays and I have been mystified by how the water is ingressing, just as you are. I drilled a few holes in the steel flooring to allow the water to escape and not pool, but have never been able to completely eradicate it no matter what I have done.
If the large cross-coach bay is wet, then empty it out close one bay door and have someone gently spray water in the area of the door, starting low and working up. Be gentle with the water. You are trying to simulate rain.
We had an issue on our '92 that took a few years to finally track down & fix. We would have a damp area in carpet by mid entry door after being in coach for a few days. Then it would dry up. Looked behind galley bottom cabinets & found some water on floor next to wall. Looked like it was coming from shower pan where shower wall dropped into it. Had Newell cut & silicone some pieces of styrofoam insulation to seal it up. Seemed to be a lot better,but! Kept seeing some water that would pool in a container cover in bay under coach. Not a lot and just once in a while. Year before last had a complete failure on ice maker fill tube. Not the pressurized supply line just the open ended fill line. When I replaced it I noticed it appeared to have been cracked for a long time. Water problems have finally disappeared. I think the reason we saw water after first few days of use was because ice maker worked a lot to get ahead of the curve. Then cycles slowed to the point that leakage was caught &n evaporated in the drip tray. Still not fully sure if shower was a contributor or not. My point is it can be a frustrating thing to track down some water problems.
I stumbled on a neat trick for detecting water. Blue shop towels turn dark blue with on drop of water. I found scream tricky leaks by pacing dry towels in suspicious places. Start high and work to low. Water won't run uphill.