Moisture build up underneath fifthwheel

We did drive though rain going over Mt. Hood to sisters...that's when I noticed the dripping. Did not give it much attention at that point, thought maybe that was the cause. Came home a week later and parked the trailer. Put a new black tank valve on and did some other maintainence, took it back to storage and then noticed the dripping again. I did have about 4 gallons of water in the gray tank to kind of flush it out. At that point I thought I had a problem but then started thinking it could be condensation between the two layers or residual rain water. Probably better tear it apart just to be sure. I really appreciate all the comments I've received. I'm new to being on a rv forum. Seems like
A good resource with a lot of nice people...