Search the internet for 'Dog cooling mat' or a 'pet cooling pad'- you can get them in a variety of sizes & a few different types, ranging from those you need to soak in water for 24 hrs first, right thru to those you just open & lay out in the shade; it all depends on how much you want to spend & if you are travelling with access to enough water & space to soak things. Regardless of the type, if you have a mat or two available so your dogs can lie on them if they feel the need then your dogs will travel a lot better! The 'just spread out' jobbies are a much better choice if your dogs travel inside the cabin with you, but you might need to have a couple for each dog on hand so that you can swap them around once the one in use gets warmed up by the hot dog using it for a few hours.... It's also a very good idea to get the dogs used to them at home before you leave - just encourage them to lie on them & transfer a bit of their own personal odour onto the mat/pad so that it becomes an acceptable 'place of shelter or relief from the heat' once you hit the road & want to use them in the car or at your various camp sites/overnight stops, etc.
I've used them for our animals for many years, after graduating from the old 'wet hessian sack' jobbie (which btw, will work best if slipped over a frame with 6" or so high legs to keep it off the ground & encourage air circulation & evaporation underneath it in order to help keep it cool) and our dogs love them in the heat, especially on those really hot nights. In fact, I hafta admit that more than just the once I've shared one of the dog's mats, cos his sleeping pad was cooler than mine!!
But they do work just as well in the car cabin while driving too, and even do fine fitted in the 'dog hotel' tank bag for when I'm travelling on the CanAm Spyder!
If you are going to be spending a fair bit of time travelling, it doesn't hurt to try & teach your dogs to do their business when & where you tell them - there's nothing worse than having your dogs get too involved in all the sights & sounds & smells of a new place whenever you stop, only to have them miss out on doing those things you'd rather they didn't do inside the car while you're travelling!! And while you are teaching them things, try to get them drinking from something you can carry in the car & leave set up in there so that they can & do drink while you are driving as well as when you stop. Dogs can get dehydrated very quickly if they are travelling in a car & even more so if they are in a cage on the back of a ute & out in the rushing air, cos if they only get to drink when you stop they might not keep hydrated well enough or you could find that taking time out for drinking conflicts with doing all their other business.... And while driving with the A/c on can help keep them cool, it's also going to be de-humidifying the air in the cabin & fairly quickly de-hydrating your dogs - but it is arguably better to do that than driving around with the outside air blasting them thru an open window with the high ambient heat from outside, no matter how much they'll get off on the overload of smells.... so they will
benefit from having access to drinking water while travelling along, IF you can teach them to drink without slopping!