Yes, it is extremely important that there is no connection between interior of the coach and the "combustion part" of the furnace due to Carbon Monoxide.
A quick test is to find a smoker and have them blow smoke in the outside furnace inlet while you turn on the furnace. If you smell any cigarette smoke, repair before using.
Depending on what gasket he is talking about, wonder why he did not replace it while in there???
BTW, I go around a CG looking for a smoker to do this exact tests on both furnaces once a year! They always seem to get a kick out of it.