No, people ship snakes every day. And oftentimes, it's "illegally" shipped via UPS and subject to horrible temperature changes, etc. You can definately transport your snakes safely in your own vehicle!
My main concern would be making sure they're each in seperate, escape-proof containers. Rubbermaids are a good choice, but you might have to use bungee cords to tighten the lids in the middle so no one can escape. An escaped python (especially the big guy!) could be completely disasterous in a moving car.
If it's going to be warm/hot during our trip, I would be concerned with them overheating. Definately don't leave them in the car with the windows rolled up.
I have never driven cross country with snakes, but I have had them in the car for a few hours. I'd make sure to keep an eye on the temps in the car (backseat?) to avoid overheating them. I don't think heatpacks or pads should be necessary if the weather is warm enough.
Hopefully others will have some advice or personal experience to draw from, and advise.
-----
4.1 Ball pythons
1.0 Boa Constrictor Imperator
0.1 albino Cranwell's horned frog
1.1 breeding Clawed frogs
1.0 black kittycat
3.1 Oscar cichlids
Also have fancy goldfish, african cichlids, and rats. And 1 Paint horse mare
