We have a 2003 FDTS40. 74777
The fresh water tank filled slowly and the front vent would siphon 10-20 gallons after stopping the fill.
1. I did the SAME AS noted above and put a valve on the front vent down by the propane tank.
2. I added a T and piping into the tank at the connection to the pump. I have a valve and hose connection at the inside front edge of that compartment door.
3. I marked 1" marks on the tank from the bottom up. These represent 10 gallons of water. I can watch the water level there and shutoff the front vent, then the water, and then the hose water outside bib.
WHen the water reaches the 90 level I reduce the water flow until full.