They are different size, so get the one which is essentially the original one, the Atwood. It will fit right into the original hole. The gas line location will need to be changed, as the new Atwood has the gas line coming through the side, not the floor like your original one. Most just drill a new hole in the case for the floor installation, but if you wish to use the side location on the new heater, you will need to change some gas piping around.
You can get an original pilot light one, an electronic ignition one (DSI) or one which also has an electric heating element plus the DSI. I recommend the DSI unit, much nicer to turn the water heater on and off with a switch than go outside and light it. You will need 12 volt
power for the DSI unit. If you go the full woot with an electric heating element, you will need to do both 12 volt
wiring and 120 volt wiring with a new 120 volt circuit.
I hope that helps some.