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.