No experience with the particular trailer but had the same problem in the Boler
. We took the heater out during the restore and replaced it with one the same model that actually worked. When we went to put it in we ran into the same difficulty. I think the original one was forced into place and when we removed it the cabinet relaxed back to normal shape.
The "new" heater stuck out about an inch. We ended up making spacers out of a hard plastic we had laying around to put under the front flange. We painted them to match the cabinet and it has worked fine. It is really close to the sink plumbing but that has not been a problem either.
I would not want a heater installed in a front wall, I do believe that when towing rain and dirt would come in the vent directly into the heater.