old Atwood water heater help

Thanks for the help with this. I finally got the trailer out and camping. While sitting in my yard I was able to light the water heater and it ran fine. When at the campground, we hooked everything up and lit the pilot. The heater took right off. It sounds like a jet plane when running but other wise it worked fine, I thought. About 15 minutes later I go out and cannot hear the jet plane any longer so I take a look and it's out. I re-light and back normal for another 15 minutes. I turn down the temperature setting a little and watch it this time. With the main cover open, it never goes out but when I close the cover, the whole thing including the pilot goes out after a few minutes. I checked for air restrictions, there are none. Everything is clear. Why will it only run when the cover is left open? (there were no leaks and the lines were purged of all air)