We bought our Four Winds 28Z last fall (which is closer to 30ft) and it has a 13,500 BTU AC. So we will find out this summer if it is adequate (we live in Michigan as well).
Before we bought the motorhome, we owned a Carver 325 boat that had both a 16,000 BTU and a 10,000 BTU AC on board. The combination of the two AC units kept the boat cool enough. But one season the 16,000 BTU AC quit working, and the 10,000 BTU unit was not enough to keep the boat cool, even along the shoreline of Lake Michigan.
The 10,000 BTU unit would keep enough to cool the boat in the evening and early morning, but noon to nine PM, it was not enough.
So as I compare that to my RV, I am still thinking that 13,500 BTU will be enough at least in the north, because the boat was a bit larger (32ft long x 12ft wide), and had no insulation in it, and had a huge amount of glass for the windshield that angled up - making it more efficient in "catching" thermal energy from the sun.
So I am hoping at least the smaller RV (in sq ft) and having insulation will help the 13,5000 BTU AC enough to be OK.
Unfortunately, they are predicting a much colder summer than normal for the upper midwest, so we probably won't even need AC at all this year.