What heater brand/modle do you have? The newer 6 gal atwoods are still flange mounted- the 10 gal (which looking at a trailer probably was) has a bit different pan mount. Its possible your old door would even transfer over if you wanted- on the 6gal. If Suburban I dont know.
Atwood manualCovers 4,6&10 gal water heaters
Your exact modle number may no longer be listed but an equivalent available. An email or phone call to Atwood or one of may retailers should get you there. When I had mine out a while back breifly thought about changing it out-inside starting would be nice but she still heats water.