If I could get my RV builder to put a "window" air conditioner into a window cutout in the trailer, instead of buying a roof unit, should I? I expect the drag would be lower.
The brand I am looking at has trailers as narrow as 6'6", 7', and 7'3". There are ways to reduce the drag.
The "drag reduction" won't be enough to offset a window unit from tearing up the wall if a standard type of "window A/C". You might look at split units, but the cost, complexity, weight and problems with service are not likely to offset the ease of installation, service and replacement of a standard RV roof unit.