If most of your camping is not in the south in the summer, the 13.5 may be the best choice. Again, can be run with generator (get the hard start capacitor). On the other hand, if you will be spending time in my neck of the woods in July, you'll need the 15K. I have both in a 27 ft. box and find I can use the 15K to cool things down quickly and later on use just the 13.5 if we don't want the noise in the living area. The other night, just after Christmas, it was 82 here, I was adding a surround system in the TT and turned on the 13.5, it cooled nicely. But then this was December after all.