Our truck AC Cools the complete inside of the MH if we keep a vent in the rear bath open some. It pulles the cool air thru and its nice inside even at temps over 110 degrees F. as long as we are moving. The roof AC is ducted and does fair but not much for the cab area so cab becomes hot. Cab AC does so good we find no reason to run both. As far as fuel difference by running one or the other I really have found little difference. For our rig using the cab AC while traveling gives us far more comfort but really may cost us 20 or 30 cents per hour more.