Jeff-
Member
Considering that the 9,000 BTU mini-splits I have installed are perfect for the 19 and 21 and many have been happy with the 9,500 BTU Houghton rooftop units even in the southern states you should be fine around that size. If I were you I would order the trailer without the oversized jet engines made by Coleman and Dometic and have a Houghton RP-AC2801 from RecPro installed yourself. They are one of the smallest capacity rooftop A/C's available and finally seem to have an acceptable noise level.
https://www.recpro.com/rv-air-conditioner-low-profile-9-5k-quiet-ac-with-remote-control-non-ducted/
Yes that is the best solution since the noise level is acceptable and no modifications. I like a little back ground fan noise as it helps me sleep, but not the noise of the units you mentioned by Coleman and Domestic. I would take the noise over being hot any day but now that there are better solutions why live with that.