It sounds like you still have the water heater bypassed for winterization.
Normally, the water will be pumped from your freshwater tank or provided by a pressurized hose hookup and ran around your RV. The cold water will enter your water heater at the inlet, fill the tank, and exit the WH outlet, and on to the hot side of your fixtures.
In bypass, a series of valves and a bypass pipe are used to cut off the water heater from the water system to make it easier for winterization.
So, make sure your valves are turned properly so that water will enter and exit the water heater properly. Turn the hot side of a faucet on and let the air escape from the system as your water heater fills. When you get a solid flow of just water and no more burping, shut off the faucet. Now, you should be able to successfully turn on your water heater.
Watch:
https://youtu.be/nHPsMYYeXGc