I know the NT22a was recalled. I would guess it applies to all. The problem was the rubber crossover tube between blower and combustion chamber disintigrates causing carbon monoxide to be blown into the coach. Plus parts are extremely difficult if not impossible to come by and get someone to work on them because of liability. Plus there just plain a pain in the @#$ compared to newer ones. I replaced mine with a NT30. You can find lots of info on replacement in a search. Basicly you build a base for the NT30 which will connect up with the NT22 ductwork. I did mine by modifying the NT22 cabinet base. If I can do it you can.
Sail on silver girl. Sail on by. Your time has come to shine.