RV Tank sensors are relatively easy to troubleshoot.
Here is the troubleshooting manual for the most common type. The indicator inside your unit might differ as to the display, but the mechanics are most likely what you have.
A signal wire from the Fresh water tank may be shorted to ground. This may happen if the wire has broken and is touching the frame of the camper.
Lou, Laura, & Freya the wonder dog
2008 GMC Sierra 3000HD Crewcab SB Allison Duramax
2019 Flagstaff 8529FLS - Pullrite 3300
HAM CALLSIGN - KC3FFW