You will have a great time! DD1 is a shocker sleep wise and has been a bad car traveller too BUT on the 3 occasions we have been on road trips she has had a ball. Seriously it has been the only time she has slept well and had good day naps. I think the change of scene and having a relaxed mum and dad (also no daily routine to stick too) made the difference, possibly saying a bit about us, lol.
We planned heaps of stops in the car - centring around food and a bit of a play/run. So we'd stop every hour and a half or so. Lots of snacks in the car - so things that are little and not too messy but spread out. Dried fruit, popcorn, lots of new packets of stuff for the novelty value.
We did lots of singing - she loves Burt Bacharach and has acquired her dad's love of Guns n Roses as well as her music. Pointed out all the different things out the window etc.
A magnetic drawing board is good too.
Have a great time - just try and go with the flow - it sounds great!!!!!