28 footer is a single A/C enough for hot summers?

4000 is the listed minimum for a single air. Two would be marginal if they even start. But the new A/Cs are getting better so maybe a longshot but not impossible. Maybe 2x 13.5 high efficiency units. Or a 13.5 and smaller 9 or 11k BTU. Good luck.

If the RV is well insulated, parked in the shade, and you have a superior installation the single air should be fine. Reality is not likely as temps go 95+ if you want a steady 72 inside.