Attwood 8500 furnace

I would NOT do it. Taking the already hot air from the furnace and adding electric heat would super-heat the air and potentially be a fire hazard.

Why not just use a small electric space heater that plugs into a regular outlet?

I was looking at adding Cheap Heat electric to mine but they don't make one to fit my Atwood 8535

Since I added an electric fireplace with 1800 watt heater in the space previously occupied by a drawer and shelf for a radio we haven't needed to use our forced air heater. It's amazing how quickly the electric heater takes away the chill and keeps the 400 sq foot 5th wheel toasty.

Many of us use electric heaters with the RV furnace kicking in when the electric heaters can't keep up. Electric heaters are quiet, don't require special wiring, and are inexpressive. I recommend a Vornado as they move alot of air.

for vornado had a recall in August for possible overheating make sure you don't have a recalled one

Atwood don't think much of "Cheap add on Heat Strips"
CheapHeat info Notice.