I had a similar problem. Finally found the joint out of the holding tanks was seeping. Got that fixed and all was good for a while. Then it started again. A little water settling in the underbelly. Eventually pooling up to be a couple gallons in there. Found out the tanks were fine. No leaks there. But then found it was a leaky vacuum breaker, that goes to the black tank flusher. They hide those things...so hard to find. Found it behind the faucet valves for the shower.
The point is your leak could be any one of a myriad of things. But is sounds like a leak, for sure. You should not be seeing noticeable condensation down there. Check any plumbing connections. Water pump, under sinks, around water heater, flush valves, city water connections, etc. A slow leak is hard to find. Place a piece of cardboard under neath any connection, and go back later to see if any evidence of leaking.