Not a dog owner, but I have a converted T5 camper so can offer some advice. The van as is when converted will get very hot in the sun, even with the front windows cracked open some, shades all round and an insulated van. However I would imagine with a pop-top (with top vent/mesh windows) and tailgate cracked open a bit (with devices listed above), it will greatly reduce the temps in the back. I would definitely advise finding someone with the same setup you plan to run and see what their van is like as standard they get very very hot inside in the sun.
Chrisl – hence the initial question about AC that can be left running to keep it cool.
Other suggestions we’ve had from other forums include thermostatic roof fans to create through drafts and things like that.
We’re at the very early stages of looking into this so wanted to get practical ideas to work out if it was possible.