Not a poorly insulated bus but a poorly insulated RV.
We have 1 13,500 Coleman TSR rooftop RV air conditioner unit. It is in a 22 ft Class C. When we need to run it, we usually turn it on in the late afternoon on high for a little while (hour or less) to get the temps down then switch the fan over to low and the thermo is usually set fairly warm. We are not into AC. We do not set it so cold that we feel like curling up and dying every time we step outside. We have spent the summer in SW GA (near Albany), and NM (near Albuquerque). I coated the aluminum roof with Elasto Seal/Snow Roof http://www.snowroof.com/catalog.asp
. The temp drops are fantastic and it was fairly cheap and easy to put the stuff on the roof. Only downside to it was I wore long pants and rolled it out on overcast mornings... I still got a sunburn on top of my feet and almost up to my knees from the reflection once I put the first white coat on (I put 2 coats on).
Hope that helped at least a little.