To add what Pete said, we have 3 brand new 15K BTU roof airs and on the way thru Tennessee I lost my generator due to a $16 start capacitor. All we had was the dash air for 2 days on the road. Now we got a little lucky, it was only in the mid to high 80 deg's and we made out ok with just the dash air while on the road. We stopped at a campground and ran the roof airs on 50 Amp shore power for the night. I have never had a bus with over the road air or cruise airs so I can not compare them to roof air. Right now I seem to be making out OK with my 3 roof airs, but they do run a lot when the temps get over 90 deg. or so.