Fuse

So a few things here. I have never heard of a “tank level screen” that monitors the level in the water heater. So if that’s what you are trying to do you’re going about it the wrong way. 
To determine if the water heater is full, turn on any hot water faucet. If a steady stream of water comes out, the WH is full. If not, air will come out until it is full. If the WH is full, go ahead and turn it on.
As for the tank level gauges, they monitor the level in the fresh water tank (this is different from the water heater tank), the black water tank, and the gray water tank. They are typically unreliable anyway and shouldn’t limit your camping ability.
It sounds like the level gauge screen may have quit working but it’s replaceable. You would need to pull it to determine what you need to replace it. That’s a whole new discussion from turning on the water heater.