As GARY said you take cover off AC on roof 4 screws have to be removed. There is no access to AC from inside your coach. Inspect for critters in fan drums or just maybe leaves or pine needles.
Your filters are located in ceiling grill vents, foam filter will be on left side looking toward rear. You place you fingers through vent grill louvers turn vent and pull down as you turn. Vent grill will pop loose and foam filter will come with it. This link will
show you the filters and how to clean. Scroll down the link for view.