Ken has a very good point here, OP. Our coach IS 30A. Even with a single A/C, that 15000 taxes the power supply and causes low voltage. IF the 13500 you now have is/was cooling OK (by that I mean to your satisfaction) then replace with 13500. Add heat strip if you want, make sure the new "upper" (the roof part) will connect to your current "lower" part (the interior ceiling part) and choose the Make/Model that matches those requirements.
ANOTHER reason NOT to upsize: An oversized A/C will cool the coach so quickly that it cycles out before it's removed enough humidity for you feel good. The coach will be CLAMMY! We can prove it. We have to run a dehumidifier to keep the dampness down. Our coach can be 100* inside and comfortable in 30 minutes and that's nice. It's also a ductless unit which I believe sustains less cooling losses. Still, we pay for that cooling power in electrical demand, humidity, and noise. That 15000 has two settings: Very Loud and Even Louder. A ducted version might not be noticeably louder than a 13500, but this ductless 15000 is a good deal louder than our older ductless 13500.