Our experience this year: We usually traveled from ~9AM until 2-3PM and the trailer would get quite warm even with both roof vents open (MaxxAir vent covers). After connecting the electric in a CG, our 13,500 A/C would run full blast (cool air shut off in the bedroom and bath) and would slowly bring the temp down. Most occasions we had little/no shade in the CG, so we covered the skylights and windows on the inside with the reflective 'windshield' sun blockers. The A/C would reduce the humidity noticeably faster than the temp.
After sunset, the 13,500 A/C did fine. Near bed time, we'd open the bedroom and bath A/C vents. At bedtime, we'd close the bedroom vent (too much cold air!
) and open some of the main area A/C vents.
If we were in a CG for multiple days, the 13,500 A/C did fine (~75 deg) as long as we turned it on before
the whole unit, inside and out, became very warm and humid.
Bottom line: IMHO, if you stay in a CG (especially in the shade), the 13,500 A/C might
be enough for your 28BHBE. I haven't experienced FL in summer, in the sun, in a TT, but I suspect one 15,000 A/C (which is only ~10% more than 13,500) would be not
enough to maintain 72 deg. in worst case situations; especially if you have traffic in and out the doors - kids or dogs - on a regular basis.