AC keeping back and front rooms cold?

I had that problem with my previous 5th wheel, so, for this one I had them install a second a/c in the bedroom. Guess what the rear a/c cools the bedroom just fine in this larger 5'er, with the same size a/c unit. I read on hear that some of the ducted a/c units are not sealed well between the unit and the ducting, and you lose a lot of cool air into the ceiling. So, first I'd check that the dump vent is fully closed, then check to see if you're losing air around the ducting, both ends, and possibly check for some obstruction in the duct. JMHO