I bought a used MH and the first time I filled it up there was a water fall under the coach.
Took 4 techs to finally figure it out it was the vents because it only happened if I passed the line in the tank where they were located when filling up with gas. Keep the gas in the tank below no leakage.
Found out the actual 'vents' themselves had come loose! I had them repaired.
Fast forward 4 years I hit a pothole (thank you Atlanta) and the vents came out again. I can fill the gas up to only a certain point, past that where the vents are located and it leaks.
Check the actual vents. The gas can be leaking down towards a line and actually be coming from a loose or lost vent.