I added a piece of 1/2 flexible tubing to extend the arch of the vent/overflow 12-13" above the floor. This put the top of the arch above the filler hose inlet. There was enough space under the dinette seat to do it.
After the fix, the tank filled until water began flowing from the filler inlet on the side of the trailer then stopped. During this time, about one gallon of water discharged from the vent/overflow at a moderate rate. This is similar to my old TT in which the vent/overflow hose was connected to the screened vent hole by the filler inlet instead of through the floor and underside.
Before the fix, the tank filled up and immediately gushed about 9-10 gallons of water out the vent/overflow tube, then probably more on the way to a campsite from the water spigot as many have noted on this forum.
I'm sure that installing a valve at the end of the vent/overflow as many have done would also work. I chose this option because I want to maintain the principle of the original system and there is a big caution box in my owners manual regarding closing off the vent tube.
Thanks all for the advice and comments.