Make sure that all of your service work is done at least two weeks before you leave. If anything is done wrong, you want to find that out at home, not on the road.
I’d guess that your oil change interval is 7,500 miles. I’d stick with that. These will be easy miles on the engine, overall. Changing it more often just gives more chances for something to go wrong on the road. If you’re still concerned, put a synthetic oil in. Make sure you carry an extra quart of oil in the car.
Are you still on your original battery? If so, I’d bite the bullet and replace it now, as you’ve gotten most of the life out of it anyway.
You definitely want your timing belt (and water pump) done before you leave, of course. Having it snap would be a disaster.
Do you have cloth seats? If so, considering the amount of eating that will probably happen in the car by all of those people, I’d consider a Scotchgard treatment. A hand-held car vacuum might be nice to have. I’d bring plastic garbage bags, paper towels, and lots of moist towelettes.
What are you doing about music? Do you have satellite radio? Do you have an iPod input? If not, you can probably have an aux input of some sort added.
Do you have enough sockets for everyone to run and charge their electronic devices? You might need some splitters and extension cords.
One of those thermoelectric coolers might be nice if you have room, although you’d have to watch that you don’t overload your electrical system with this plus the other electronic devices.
You could save some money by using the GasBuddy smartphone app or web site on the trip.
Can your car carry a full-size spare tire instead of the compact spare that I assume you have now? If so, that would be handy if you get a flat tire in the middle of nowhere. You could also consider carrying one of those cordless tire inflators. You definitely want a tire gauge (preferably a good dial-type one for accuracy).
I assume you already plan to carry at least a basic tool kit. A small first-aid kit wouldn’t be a bad idea either. Make sure you have a good LED flashlight with lithium batteries. (I’m partial to Fenix, myself.)
I’d probably have an A/C shop check over the A/C system. I’d want it operating at peak performance with six people in the desert.
Do you have a credit card that gives you points for gas purchases? You could build up a lot of points during a trip like this.