There should be a small flat piece of metal with a hole on the outside on the cover, see pic below. Lift it up and turn it so it's lined up with the slot in the cover and the door should open. You might have to pry the top or side carefully to loosen it.
Make sure you have water in the water heater by opening the pressure relief valve a little and close it again. Stand back so you don't get soaked. If you don't have water in it the bypass valves aren't turned to the correct position. For electric see if here is a switch on the water heater, if so turn it on. Then go inside and turn on the electric water heater switch. For gas make sure the LP tank valve is open and turn on the Gas or LP water heater switch on the inside of the camper.