Find water near you destination and fill the tank again. If it overflows almost immediately it is just a gauge issue. If it takes a lot of water then it is probably being siphoned out going down the road, a number of people on here have that issue.
I have a control panel with membrane switches of many functions and the gauges. It is terrible. I filled the fresh water the other day and it is reading 1/4 or less. I have had both the panel and relay box replaced twice, worthless.
TT 2017 Flagstaff 831CLBSS Classic Ultra lite
TV 2017 F150 6.5' bed 3.5 Eco-boost
prior TV 2014 Silverado 2500 HD