The airflow return on our furnace is under the refrigerator,, right beside the stove/oven, but that space under both those areas where the furnace is located is open to the hot water heater, kitchen base cabinets, and a number of cabinet doors that open all along the cabinetry. There'd be no way to effectively seal off that one area to force airflow through a filter. Besides, in all my years of RVing, I've never had a problem with a clogged heat exchanger on an RV furnace. Burned out/rusted out exchanger, control board malfunctions, lots of empty propane tanks, dead batteries, dirt dobber nests in the exaust, but no filter related problems. I guess our RV's are just "cleaner" than our houses