In 1992, I did a 6-week road trip on less than $1000, traveling with one buddy.
Of course, the type of trip we did made that possible. We camped almost all the way, and we already had all the equipment, so our 'lodging' cost was only about $10-$15 per person, per night. Most campground fees around the country are between $20-$30 for a site. And we weren't really roughing it -- a lot of the campgrounds we found boasted swimming pools, stores, nice showers, even cafes.
Food: We depended as much on ourselves as we could. When in a city, we'd stop at a grocery and load up the coolers. We ate well, though! Veggies from the salad bar and dip (like hummus)travel well. Lunch meats and cheeses make good 'roll-ups'. Bisquick was our best friend. We had Bisquick pancakes and biscuits for breakfast about every other morning, along with coffee, which we made at our campsite. On other mornings we had granola bars or packaged oatmeal mix. We always had a few pieces of fruit around too. We ate most of our meals 'out' at mid-day as suggested above, and snacked at night on cheese, crackers, pepperoni, and stuff like that. Buying so much of our food at the grocery left us lots of spare change for splurging on pastries at local bakeries and treats like homemade ice creams and roadside food.
AAA was very helpful. We got a Trip-Tik before the trip, and even though we didn't follow it exactly, it was invaluable. The level of detail on a Trip-Tik far supercedes anything you can find on a web site or McNally atlas. Trip-Tiks show what services are available at each highway exit, what hotels and campgrounds are nearby, and highlights local attractions. It only costs around $50 to join Triple-A, and if you get into trouble they will come out and fix your flat tires, bring you gas, or arrange a tow. Many cities around the country have a Triple-A office where you can cash travelers' checks, pick up maps, and even find great discounts on local hotels and restaurants. It'll pay for itself if you join and take advantage of what they offer.
We concentrated on National Parks and the outdoors. That was cheap. We bought the "Golden Eagle Passport", which permits admission to one carload of people to ALL National Parks for one year. And it's only $50.00. So if you're going to visit at least 4 or 5 of the parks, it's well worth it -- most park admission fees are $10 or $20. Once in the parks, there's a lot of free activities. Ranger talks, hikes, movies, campfires, etc -- on all sorts of topics. Hiking is free. Hanging out at Nat. Park campsites is a great way to meet people from all over the world.
Talk to the locals. Don't be afraid to stop people on the street and say "We're looking for a great, inexpensive place to eat" or "Where's a good place to go out for drinks, that's not touristy?" Most people are really helpful with this.
Hostels are sometimes not that much cheaper than motels, as noted above. BUT they often come with interesting bonuses. They attract adventurous people, often younger people, from all over the world. You ALWAYS meet people at a hostel. Sometimes people who have been there a few days can give you great tips on things to do and see. The hostel staffers can inform you about local 'budget' deals like 2-for-1 nights and such. Some hostels offer bikes to borrow, and have libraries of books to trade.
Check bulletin boards in every town you visit, especially college towns. There might be free stuff -- dances, concerts in parks, movies on the beach -- you never know.
Festivals and street fairs can be great entertainment, too. Watch for posters.
Have a great time, you guys. You will remember it forever!