Hi! Welcome to the forums!
I prefer to use slide toppers to keep my slides clean and not have to worry about debris, otherwise the only way is to check it before you bring it in. You could put an inspection mirror on a pole to take a look up there.
Not sure about the AC/Furnace issue, which thermostat, furnace and A/C do you have?
No clue on covers... I am building a 40'x40' barn as an RV cover.
Maybe on the FF, it will fit in any 14"x14" vent opening, the trick is getting power to it. Pull the interior trim off the vent you want to use and see if there are any 12 volt power cables up there, or perhaps you can pull power from a nearby ceiling light fixture.