Dennis King and others . . .
I have a GT5 and had the same issue with the fresh water tank siphoning after water started running out the overflow.
Problem solved . . . eliminate the siphon action.
I removed the tape-screen from the bottom of the overflow (save it for later use).
I then inserted a lenght of 1/4" vinyl tubine (Home Depot) into the overflow, far enough that it reached the TOP of the first bend in the overflow line (or maybe just a couple of inches further), leaving about 1" of tubing extending below the bottom of the overflow line. I use the same tape-screen to tape the 1/4" vinly tubing in place and keeping the rest of the overflow tube screened as well. Water MUST be allowed to run out both the OEM overflow tube and the 1/4" tube. The 1/4" vinyl tubing will break the siphon.
How did I know the tubing was at the TOP of the first bend? I removed the cover inside the basement bin that allowed me to see the overflow tube, and the 1/4" tube moving inside it. I could also tell by resistance when pushing the 1/4" tubing into the overflow tube that I had reached the TOP of the first bend.
When filling the fresh water tank, I still have some water run out indicating the tank is more than full, but it runs out only for a minute or less, leaving me with a completely full tank.
2017 GT 31L5