I just finished up a modification that should stop the siphoning. I ran a 4'-0" loop of tubing (out of the 90-degree fitting at the top of the tank) over the top of the tank, and back alongside the tank fitting. Then I connected a 1/2" brass 90-degree compression fitting (having drilled a 1/8" hole in its top portion for an air vent). I used a 1/2" copper pipe strap to clamp the tubing to the underside of of the motorhome's floor pan, holding it up higher than the tank fitting.
That being done, I reassembled everything and 'slowly' filled the water tank up to the top of the 'Full' water tank sensor. That left the water level about 1-1/4" below the top surface of the tank.
A series of photos of the modification can be seen beginning at this link:
I realize that I may still lose some water when making a right turn or stopping quickly, but hopefully it will be minimal. Thanks again for all of your help
Regards, Alta Sierra