I'm no HVAC expert but, if it were me going to see this van, I'd fire up the AC, and set it 10
degrees (if possible) below whatever the outside air temperature is. I would think it should
keep running and trying to get the inside air temp down to that setting (if the thermostat is
working). Then after a while, maybe 5-10 minutes or more, I'd just put my hand in front of
the blower/vents, and see if it feels significantly colder, than the rest of the inside air.
Most class B sized rooftop AC units are the recirculating type, I think, so they shouldn't be
pulling in very much outside air, as far as I know. You could check and see if your particular
unit/type has a "fresh/recirculate" control, like your dash air conditioning. If it does allow
outside air in, set it to recirculate first.
That's what I'd do.