Can you take the old one out and carry it into a real auto parts store such as NAPA or Carquest? They should be able to match it up to one in their spec books. If they can't, ask them to call their filter supplier, who usually has application data going back forever. Most NAPA store clerks are willing to make this level of effort to get what you need.
Former owner of 2004 American Tradition and several other RVs
Home is West Palm Beach, FL