Seriously, you need to drop the belly panel and investigate.
While it could be you drove through pouring rain and the winds sucked the water up through the belly vent, most likely you have a leaker.
With everything exposed, pressurize the water system and look for leaks.
Finding none, fill the water holding tanks full and check again.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW