I'm just guessing and throwing out some ideas. I think basically that it's a lot of cubic feet that the AC is trying to cool. I have heard a lot of the same issues from other TH owners. A couple of ideas to try and help- there are vent pillows that you can buy like these:
http://www.campingworld.com/shopping...-cushion/26552 I've seen these at RV stores & Walmart RV sections. I suspect everything you can do to help insulate your RV is just like insulating your home.
With the AC in my bedroom it gets the coolest the quickest. I noticed that on the AC it has several vents that I can close to help force the air further. They also cut a round vent in the bedroom and the bath. I cut a piece of cardboard and plugged the round bedroom and bathroom vents.
I think you just have to do what you can to get the air where you want it.