Agree on the 15K. Our HTT had a 13.5 and was great with one exception....camping in Vicksburg, MS in July and the only shade tree was sorta within sight. Miserable until after sundown. When we made the change to our 26' box TT, it came with a 15K. Never had a problem in TN, GA, trip to CO in July....always very comfy with or without shade. Obviously, shade is a help in either case, but I would not go with the 13.5 in our toasty/HUMID south. Yes, include the upgrade as part of your offer. Good luck and stay cool.