The only moves I've ever done are small.. about 200 miles. I would usually just pack the snakes in smaller tubs (16qts) and put tons of shredded newspaper in there. Then I'd usually put them in the back wrapped in a blanket during a summer move with AC on or in the front seat with the heat indirectly blowing on them. Either way I've never had problems. But for a move that'll take 2-3 days like you said, I think your best bet may be shipping them once you're established. Or you could bring heat tape for your snakes so that once you get to a hotel, you just bring your snakes in and let them have proper heating while you sleep. Then in the morning, pack em up again and do the tub thing I was talking about. Repeat.

For a van, why don't you try hanging a blanket or two vertically (behind you, the baby, the dog) so you can keep AC on in your area, and the snakes shouldn't get too much of it. The snakes can simply be kept in their own 'climate' in the back.

Just a couple thoughts...