In our FR Flagstaff
It is behind the rear basement wall with the water pump.
Look below where the city water connection is located; but on the inside or the wall.
I used a box cutter to remove about 18 inches of rear wall luan to get better access to the filter. It gets replaced every year and is a nightmare to gain access.
Lou and Laura with Bella - German Short Hair Pointer
2008 GMC Sierra 2500HD Crewcab SB Allison Duramax
2010 Flagstaff 8526RLWS - Superglide 3300
HAM CALLSIGN - KC3FFW