Matstam13 -
Oh, this is another one of those "features" that depends if your RV was built on a Monday/Friday or some other day of the week. If your Alpine is like DocData757's, then you have some filter material behind those return air vents that is a pain to get to. If your Alpine is like mine, you don't have any filters. But in my own manner, I made a modification, or add-on, that gives me a filter.
See:
http://www.keystonerv.org/forums/showthread.php?t=5683
Ron
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2011 Alpine 3640RL (Beauty)
(Gone! Now replaced by Beauty3)
2016 Ford F-450 (The Beast)
Diesel 4x4, DRW, LB, CC
Comfort Ride Hitch