If you are starting from scratch, you may want to consider a hybrid van. The climate control systems will be electric and built-in. Right now in the US that pretty much limits you to the Chrysler Pacifica and the Toyota Sienna. Ford has hybrid Transit vans in Europe, but they don't seem to be bringing them to the US.
Hybrids can run in "camper mode" to keep the AC running off the battery when not on the road - they can also provide heat in cold weather. In the Sienna when the battery runs low the engine will kick on for a while to recharge the battery (like the Prius). Note that in the Pacifica camper mode requires a modification to keep the system running overnight - but a plus is that the Pacifica is also available as a plug-in hybrid with a larger battery.
Kia may be bringing a hybrid version of its Carnival van to the US next year.
Pacifica climate control mod:
https://relentlessoptimizer.com/pacifica/circuit/