An RV furnace does have two fans on one motor, one for moving combustion air/byproducts and one for moving interior air. They are sealed from each other, but powered by the same motor and operate at the same time. Residential furnaces are different in that regard.
Is the new motor also correspondingly higher power than the old one? If not, I suspect it would end up operating somewhat below its rated RPM and running somewhat hotter for that reason and so may not have the greatest longevity. If it does run significantly faster than the original, it may—or may not—be necessary to readjust the burner for a proper flame (or maybe even constrict the burner exhaust vent slightly). Too fast would be preferable to too slow, which would simply not activate the sail switch and cause non-operation.
If there are other parts of the furnace that look like they're on their last legs, maybe it's not worth replacing the motor since many key parts are no longer available. (On the other hand, if the burner and heat exchanger and so forth are in good shape, I think it's silly not to give it the old college try.)
