Have I thought of everything ?
Yes I guess so.
But have you thought of cleaning the coils on the two existing units? I've been out west and the killer south and 2 do the job no matter how long the coach is.
I have to admit after venturing out of my element of the Midwest I was amazing how those coils get fouled up with stuff. I had them cleaned and could not believe how much colder that AC unit worked.
I now have the coils cleaned as a routine maintenance when in the south.
Pillow inserts in all the vents and cover the shower skylight drastically cuts down on how much heat comes in too. Just saying.