IF Tiffin was using the current Nippon Denso AC units in 2007(It must have the dash controls in the linked pic), it is outside on the Evap housing. Current Tiffins use the Denso AC system. Look at the white object in the pic on this link. That is the cabin filter. Doug Older Denso systems do not have the cabin filter densoheavyduty.com/hvac-systems