Nissan NV tow vehicle van conversion
I guess it's because I have spent so much time in hot/humid climates. The vehicle A/C unit would not be able to effectively lower both humidity (to around 40%) and temperature (20F below ambient or better) in the places I've lived and worked.
I've found that once an interior is humidity soaked that it takes several days of running a dehumidifier in a closed vehicle to bring moisture levels down to the point mold and mildew are not forming.
I understand the concern about appearance. I find A/S trailers with roof vent rain covers to be unreservedly ugly. The next A/C unit on my trailer will be one of the new low profile units from Coleman. In the same way, a Maxx-Air unit versus Fantastic Fan as the latter requires one of those plastic warts for use in rain.
FWIW, Wally Byam used to spec his trailers with a floor vent. The convective flow was said to equalize indoor and outdoor temps effectively.