Pretty sure that there is a pressure relief device on the other end of the water tank. You can open that to verify there's water in the tank, won't hurt anything, water should belch out of it when your RV is connected to shore water or the pump is turned on. Not much, but enough to show you the tank is full. Turn off the water heater the night before so it'll not scald any bystanders.
And I agree with the others that those valves you show in the picture should both be turned 90*. And if you have an electric element, it's likely damaged if you turned it on with the tank empty. Often they melt before they open circuit. There might be a safety device to protect it though so check for that too.