Sometimes motion of the trailer can start water siphoning out of the overflow/vent line of a full tank. Try setting out with the tank about 2/3 full and see if you still have the problem.
If that is the prolem try re-routing the line to go higher in the run to where it comes out. Alternatively, put a shut-off valve in the line that you can easily open after you've moored.
Frank and Eileen
No longer RVers or FR owners