We are living in our 298 right now as the house is under a major remodel. The temps here are getting close to 100, heat index of 110ish. I bought an LG 12K BTU unit to help cool the camper, between it and the factory roof AC it barely keeps the temp in the mid-low 70's in the living room, the bedroom barely gets below 80. My windows are rounded in the corners so rather than mess with the exhaust mounting hardware that came with it, so I ended up just cutting a piece of plexiglass that I had laying around the house. The downside is you can't run anything else off the wall outlets or it trips a breaker.
I called and emailed Keystone about adding a second Dometic rooftop AC unit, since its already wired for 50 amp. The person I called on the phone said the camper is not wired for a second AC unit, nor is the roof braced. However the person I emailed from Keystone assured me that it was braced and wired, set up for a 13.5 unit. I can get the AC unit and control panel for around $700 from Amazon so if its pre-wired I'll probably end up installing a second AC unit that way so I can use the camper year round if I like and not have to mess around with securing a mobile unit and rigging up a window vent.