If I can chip in a bit more, then I'd suggest you allow a bigger part of the budget for fuel. It is a road trip after all and the car is everything. You might consider using higher octane if possible. Slightly more expensive, but the engine runs cleaner and with slightly better mileage. Also better response in acceleration, which makes for easier driving.
Checking tyre pressures every day will save gallons. I used to run them a couple of pounds over, but not everyone will agree. I was actually given a roof-rack, but it messed up the driving and wasted fuel, so I left it behind. Not that I really needed it. I just spread stuff out inside the car with the heaviest stuff in the middle. But it's not for me to tell you how, as most of the fun was finding my method and adapting along the way. I will say one thing though - and that is not to make driving it feel like a Sunday afternoon trip to the local dump.