On my 2003 Ultimate, there are no furnace filters. The only filter is under the closet floor, and it is for the heat pump/air conditioner. The return air goes under the bed, and from there under the closet. Access is from a portion of the closet floor that pulls up. Oh, and the closet on mine is across the back of the rig. I presume if the bed were on the back wall, that the heat pump/air conditioner filter would likely be under the bed.