Furnace filter

I made a frame to accept a filter and placed it in front of the return air. (about 11x23 inches) My furnace guy gave it to me and it's washable. keep it clean and it will not hurt performance. To answer the ? no your furnace does not have a filter. What are you thinking? They cost money. We can't be spending money and c##p like that.

I don't know if the filter does anything or not but, it does knock off a couple of db's.

I've used the thin cut to fit A/C filter on my RV furnace intake for 25 years. Three different MHs and never had a problem. I went to school 25 years ago for Heating and Refrigeration and learned the importance of a filter and the proper type! I use the thin black filters with the plastic framing, cut to fit both on the A/C and furnace. In the winter the furnace is changed monthly when using the furnace just like the A/C they need to be changed at least monthly when in use. I also vacuum the area around the furnace that accessible every fall before running the furnace. A proper filter will not restrict the air flow unless it is left in and never changed just like in a home.

RV furnaces do not use a filter.
A filter would restrict the return air flow, fan can not suck enough air thru a filter and furnace would overheat due to lack of return air flow.

No filters on RV furnaces

Which is a good thing sorta......5 years is a long time not to change a filter

No it does not. Not enough fan pressure to push air through filter media