I've got a big high wall Jayco Cardinal SD (24' open, 14' box) with a 13,000 btu Coleman, and it works fine in the hottest of weather here in Michigan.
btw - it's for sale. It's a '93 that looks like new. Always stored indoors, owned since new. Nothing built today compares to the quality. Let me know if your interested - great value.
PM me if you're interested.
here's a pic.