Is this a new-to-you camper, or it worked before and now it doesn't?
The first thing is, don't try to heat the water until you are sure water is in the tank. To find out, make sure the plumbing valve(s) are set to "Normal" mode and not "Bypass" mode. This is usually located on the inside of the RV near the water heater. You may need to get into an access panel, remove some drawers, lift the bed, whatever is required based on the layout of your RV.
Next, make sure your water pump is on, or that you are connected to the city water connection so water can fill the tank once the valve is set to "Normal". Lastly, a way to check if the tank is full is to briefly pull up on the little lever on the pressure relief valve on the outside portion of the water heater panel. If water comes out, the tank is full. The reason this is important is because if you try to heat the tank without water in it 1) the electric element may be damaged which would require replacement 2) the safety system shuts the heater down and will need to be reset.
Once you know water is in the tank, it may be a good idea to press the "Reset" buttons in the outside WH panel. Then, the switch in that panel is for the electric heating element, and the one inside on your wall is for the propane burner.
Watch this video...
https://youtu.be/vk9Nt6RMHps?si=RABYksu3rmTH2rZ1