Google "rv furnace screens" ... I found quite a few for all makes of furnaces from multiple online sources, most of which cater to RV enthusiasts. (Also an listing through Home Depot.)
You may want to do the same for your refrigerator and water heater. (Ordinary hardware cloth -- window screen material -- works for flat vents, as long as they don't get too hot.)
Your refrigerator usually has an intake vent down low on the outside of the motorhome. In turn, an exhaust vent up high or on the roof. The water heater usually has two vents (high and low) on the exterior access door.
As for removing and replacing the furnace, my experience is a large vocabulary of swear words, in multiple languages, comes in handy.
Difficult parts are disconnecting the propane line and 12VDC blower motor wires. The 120VAC is usually an ordinary three-prong plug in a standard household outlet.
Physical removal and replacement is dependent entirely on how Fleetwood designed and built your particular model. Unfortunately, motorhomes aren't designed and built with maintenance in mine, so you may be in for a tussle.
The furnaces are usually bolted down and invariably there's one bolt you can't see, don't notice, or can't quite reach. So, have someone with small hands and arms available.
