Fuse

Also if you even briefly turned on the water heater with no water in the tank it will burn out the heat element. I think a replacement element is under $30. But you would have to look open the water heater door and see if you can find a tag or label with the make and model of the water heater.

There may be another switch on the water heater itself. It would be behind the access panel on the outside of the RV. Open the outside panel and look for another switch.

Also, the electrical element in the water heater is 120V, (not battery power). You need to be connected to electricity first. Secondly it would be protected by circuit breakers not fuses. So make sure you check all the circuit breakers also (they may be in a separate panel from the fuses).

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.