On an AC size for a 159 WB.
Did a quick run of the heat loos spreadsheet and came up with the numbers below -- you can download the spreadsheet and refine the numbers to what you actually have.
For 90F outside and 70F inside, I get a heat gain of 1200 bTU/hr. To this you need to add about 300 BTU/hr per person in the van, and if you plan to cook in the van add some more for that. So, maybe that gives you about 2000 BTU/hr with two people in the van.
This is for these R values:
Walls and ceiling R11
0.5 air changes per hour
The walls and ceiling could be 1.5 inches of poyiso plus paneling and air layers to get R11. This is somewhat more than most people insulate to, but if you are going to be using AC, its definitely worth it.
Windows are R3 could be a single glazed window with a snug fitting Reflectex shade inside.
Important to have reflective shades on the windows for AC. A window in direct sun can gain 200+ BTU/hr per sqft of windows -- so, 10 sqft of unshaded unreflectorized window would about double your heat gain!
These also assume parking pretty much in the shade -- parking in the sun will increase the gain for the half of the van in the sun as the van skin temp will be high.
If it was 100F outside, then the heat gain wold be about 1800 + 600 = 2400 BTU/hr ish.
So, it seems like a 5000 BTU/hr AC would be fine -- it would have some excess capacity for the initial cool down of a hot van, and it would probably run about half the time or less once it got the initial cool down done.
I think that being able to use a smaller AC (like 5000 BTU/hr) is a winner all around. Smaller/cheaper AC, smaller/cheaper/quieter(?) generator, smaller/cheaper inverter, and able to handle flaky shore power better. Maybe even run a on a big house battery for limited time.
There is probably enough margin to cover situations where you can't park in the shade or some additional miscellaneous internal heat gains.
Another thought would be to close off the cab area with an insulated curtain. This would cut down the heat gain for the large cab windows (but you still want the reflective shades on them).
If it was a good insulating curtain, this might cut heat gain by as much as 25%.
Would be good to hear from someone who has actually tried a 5000 BTU/hr AC in a well insulated van.
600 watts of solar aimed at the sun (not flat on the roof) would run an efficient 5000 BTU/hr AC on a 50 to 75% duty cycle and still be able to charge the house battery some!