Cold air return

Assume you are referring to the propane furnace. The return will be very close to the furnace itself. The air returns do not have filters on them.

If under a sofa or other area with reasonable "return area", you may not even have a dedicated return.

You might find some info here Newmar Brochures

If you have a standard LP furnace it is the wood louver cover on the inside cabinet that is where the furnace is located. If you have an Aqua Hot system it is the complete cabinet that recirculates the air thru the heat exchangers. If you are asking about the Roof top Heat Pumps, it is the same return vents in the roof on the drivers side that the filters are in they use the same return as the AC cool mode. There is NO Furnace/Aqua Hot filter system on a Newmar or 99% of RV's. Doug