The fact is that the temperature drop on new A/C's is at best 20deg F means that when you are starting with a coach at 100deg, you are never going to cool it. Now, starting the A/C up before the interior temps reach 80, and you will have no problem maintaining a comfortable level in a 27' coach with a single 15,000btu unit.
The color of the coach will be a factor too. A coach with a dark colour scheme will be more challanged than a lighter coloured one. I have no problems cooling and living in my 2001 35U on 30 amps here in Florida. Bad campground power systems is the bigger problem.
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Neil V
2001 Winnebago Adventurer WFG35U