If water supply got connected to black tank flush connection then tank would have filled and backed up thru toilet, leaked from anti-siphon device under bathroom sink (flooding bathroom etc) OR backed up so much it came out of the roof vent...raining water down from top of TT
Raining......has it been raining and leaking in via a slideout???
Fresh water tank overflowing would leak into ground thru overflow pipe
SO.....
What plumbing is close to the dinette/sofa------is water heater UNDER the sofa/close to it??
Sounds like a hand-tightened fitting is leaking!!
Water is an RV KILLER. Investigate/Locate the source.....open cabinets, storage compartments. FIND the leak and then notify DEALER of problem..
Hope you have it Insured