The spare air I have sitting in my garage is a functioning unit just as the ones are functioning on the roof. All internal fans, compressors, electronics are working.
My other option is to get a smaller footprint unit(such as a Dometic non ducted) and possibly place it the correct way in my current Kool-a-matic vent space or actually make a separate hole thus locating it exactly where I want it. I've had this notion for a couple years now...my travels are generally in areas where its HOT here in Calif and while my two airs are normally sufficient, having just a bit more on those really hot days(110+), would be nice.
On those hot days, as you walk front to back in the MH, you can feel the warm air in the area I want to place this third unit.....it might be something as simple as putting an insulated cover over that darn Kool-a-matic vent as you might for a bath vent. I'm still kicking this about, not sure I want to tackle this project but I do have some other work to do on the roof, so while I'm up there , who knows????