We previously had a 28 ft TT with a non-ducted 13.5 K rooftop ac unit. I recall camping in upper 90F temps and that unit was always adequate for keeping cool. Our ducted MoHo has 2 13.5 K units and we need both cranking at similar temps to maintain adequacy. I think the problem is that ducted units are constrained and cannot supply enough air to cool adequately, having owned both types of campers.
A suggestion to the OP: try using a fan to better circulate air in the TT and see if this helps the 13.5 K AC.
Good luck.
