1. My new fantastic fan mounting holes didn't
line up with the old vent holes (on a 2000 Casita
16LD), drilled new ones. From about 1999 on, (I think) all Casitas have wiring to the vent opening. Prior to that it's 'go fish'.
2. I Fastened the fan with SS screws
instead of rivets. This way you can lightly fasten each screw then go round again and snug em down. Silicon II adheres well to the plastic fan frame and to the fiberglass but latex caulk seems to be the sealer of choice in the rest of the RV world. I used Silicon.
3. After trimming the interior facia (or whatever it's called), I used 4 small sheet metal screws to hold it in place (see photo). Pushing the carpet back, drilled pilot holes first. I dunno how Casita
4. Without a Maxx vent cover you get better air circulation, however, folks who have camped in the rain seem to prefer a vent cover. On my last 4 rvs I've had Fantastic fans (love em) with NO cover on any of em. The latest has the Automatic Rain Sensor and it's the neatest feature since sliced bread. Closes when it rains, opens back up when it quits, or you can do it manually. Never a worry about things getting wet!