Let me guess, water drains through small gaps in the corrugated plastic layer under your RV? Mine does that during and after driving to camp. It's not leaky water lines, but the fresh tank breather that spills water as the water in the tank sloshes around. That breather is the highest point on the tank (theoretically) and is there to provide an air gap to prevent a vacuum while draining. If I fill my tank more than 1/4 full it will slosh water out the breather which is above the coroplast sheet. It drips water for an hour or more after arrival. Everyone asks...
It's a pain but I haven't found a practical solution, other than extending and rerouting the breather tube high enough so water can't escape. The idea of a clogged breather tube concerns me more than water on the plastic sheet so I removed the extension and live with it. OR travel empty and fill the water upon arrival. Good luck. David