There are many things that can cause noises and lack of maintenance is a sure way to shorten the life and invite improper operation of the furnace. What year is your neighbors coach? If it is an older unit, the noise is probably the fan motor as the bushings have worn. This can also cause the unit to not fire up properly if the air flow is insufficient to set the sail switch. You might just mention to your neighbor one cold morning that you noticed his furnace struggling to keep him warm the night before. It might motivate him to get it serviced, or if need be, replace the motor. and clean the unit.
Mike, RVIA & RVSA Certified Master RV Technician
Amy, Dr. Assistant - Roxie & Mei Ling, four legs each
2000 Gulf Stream Scenic Cruiser 450 hp & 1330# torque
06 Saturn Vue, 06 Chevy Z71 4x4 & 2014 Corvette Z51 M7