I find the front of the coach to be the least insulated. So adding length is not directly proportional since all coaches start with the poorly insulted front end. In a pinch, one AC will keep it livable at least IF I start the day with a cool coach. One AC will not effectively COOL DOWN a hot coach.
Hang a blanket from the overhead bunk, does wonders to isolate the heat loss from the front end.
I do a lot of 30A camping, so I have to do some planning and use 2 ACs for short periods as needed.
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