In MAX A/C the system is in re-circulate and taking air from inside the coach to re-cool it.
So if you do have a fresh air filter , it's by-passed in Max A/C mode.
Any chance you could post a picture of your filter , , I'm working on adapting a cab air filter to my coach.
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99DSDP 3884, Freightliner, XC, CAT 3126B, 300 HP /ALLISON 3060
2000 Caravan toad, Remco & Blue Ox.