There are several threads here about air conditioner choices. In my non-expert opinion, a lot depends on your intended use. For example, for a 28 ft a 13,500 BTU unit would most likely work unless you spend most of your time in hotter areas (Phoenix in July). We used a 13,500 BTU Power Saver model, so we could run the A/C on our 2200 watt generator. (Our restoration is documented here, if interested, https://www.airforums.com/forums/f39...er-155180.html
) We purchased ours from Camping World, but only because of price and avoiding shipping costs by picking it up ourselves. We found TheGreatly's posts on their installation very helpful (they used a 9800 BTU low profile unit, if I recall.)
As a suggestion, if you click on the forum topic to the right of your thread "Furnaces, Fireplaces, and Air Conditioning", those threads will pop up. Start reading all of the ones about Air Conditioners and, in no time, you'll know all the questions you need to ask and many recommendations.
Roy and Marie