If you consider to plan a similar trip here is some information that might be useful:
Lodging is quite expensive in the US - especially if you don't like campgrounds and prefer motels. Traveling off season can save a lot of bucks, of course. Room rates often triple in price if you travel in July / August! Use coupons for motels and hotels wherever you can. Check out roomsaver.com and HotelCoupons.com for discounts. Those offers are usually based on the availability of a limited number of discounted rooms, reservations are not accepted in most cases. Printed versions of the coupons usually are available at gas stations, visitor centers or simply near newspaper boxes on the street.
If you plan to visit a number of National Parks purchase the National Parks Pass for $50 at the first National Park on your trip. You simply show it at the entrance of every park and it is valid for one year from the month of purchase. It is not transferable by the way and signatures sometimes are crosschecked with your valid id - so letting your friends have the pass for their trip to the US is not a good idea! The Golden Eagle sticker ($15) was not worth it on our trip because we were only able to use it at Mono Lake. Yet, the National Parks Pass pays off after a few parks as the entrance fee of a single park ranges from a few bucks up to $20 per vehicle as in Grand Canyon, Zion and Bryce Canyon National Parks.
Although it is usually very simple to get around on the major highways, having good maps at hand is highly recommendable. AAA provides excellent maps for that purpose. Members (also members of affiliated organisations such as the German ADAC) can get them for free at the local AAA offices.
And of course: Never forget to carry enough water on your trips... :-)