You might not need a new AC. From Kestone forum:
PSo I've been asking questions lately about adding a 2nd a/c to our new 2016 31sqb. The single 15,000 btu has just not been cutting the heat of our Ky summer.
Before taking the plunge to add another unit, last night we pulled off all vent covers and the main A/C panel and began to clean up and better tape off duct work openings. To back up a bit, the air flow has never (3 weekend trips so far), felt very optimal. The master bdrm and bunk room just felt weak. So anyway, when we start really examining the bunk room vent we were stunned to realize that the duct for that vent continued more than 4 feet past the vent to the back of the TT with no more vents. Just flowed back there to nothing? Then, we checked our master bdrm and same story.....about 3 feet of dead space it went to past the vent with no where really to go.
After a trip for supplies, my husband blocked off the back sides only of each vent with some thick styrofoam type product and neatly foil taped it all off. Now, those two vents essentially have a dead end at the vent rather than several feet beyond them. As soon as we got it all out back together and turned it on we were amazed! Saw dust was coming out in both rooms and going everywhere. Apparently the air flow was so weak before, it never even stirred up that dust that was there from construction until we did this!
All this being said, the true test will be our next hot weekend trip. We may still end up needing another unit but this is bound to help!!