Hi,If you download the Heat loss calculator and look at the numbers, I think the heat loss from a well insulated van could be reduce by 4/5's to ~ 3,000 BTU's / HR. But probably still to much amp draw to run through the night on battery power. So no boon-docking with air conditioning without a generator. It would take a minimum of a Honda 2000?
On a really hot day it could supplement the vehicle air conditioning while driving with an alternator to battery charger running.
How long does the compressor cycle on?
The heat loss calculator is mine -- happy to answer any questions about it.
Couple of things to bear in mind when using it for heat gain (AC) are that people, pets, cooking etc. add to the heat gain. This is a plus when heating, but a minus when cooling. Heat gain per person is about 300 BTU/hr depending on how active they are.
The other thing is direct solar gain through windows. This can be a big swinger -- for a window in direct sun with no shade on it, it can be 200+ BTU/hr per sqft of window. So, well fitted reflective window shades are a must -- even better if you can shade the outside of the windows.
Of course, parking in the shade (if you can find it) makes a big difference.
One solution that involves some DIY is to adapt a small room AC to the van. Something like this one: https://www.amazon.com/Frigidaire-FFRA0511R1-Window-Mounted-Mini-Compact-Conditioner/dp/B00W2KG92Y/ref=sr_1_2?ie=UTF8&qid=1483755475&sr=8-2&keywords=frigidaire+small+room+ac
Its a SEER 11, which is pretty good, especially compared to RV AC units.
The 5000 BTU/hr should be OK for a well insulated van with good shades on the windows -- but, it might take a while to catch up if the van starts out hot.
I've seen youtube videos from people who have adapted this kind of AC.
If your steady state heat gain is (say) 3000 BTU/hr, and you have a SEER 11 AC (COP = 3.22, efficiency = 322%), then the steady wattage would be (3000 BTU/hr)(1/3.412 BTU/hr/wat) (1/3.22) = 273 watts. This would be an average of (273 watts)/(12.5 volts) (1/0.9 inverter efic) = 24 amp. Or, 24 amp-hrs per hour of operation when producing an average of 3000BTU/hr of cooling. Maybe something that would be workable without shore power, but a BIG house battery, and operating only part of the day.
Specs here: http://www.frigidaire.com/Home-Comfort/Air-Conditioning/Window-Mounted-AC/FFRA0511R1/
The spec says 450 watts when cooling at 5000 BTU/hr, so good agreement with calc above.