I certainly don't know all the secrets to an inexpensive vacation, but I have a few ideas that have helped us this last few weeks.
1. Pack a cooler. Every day we were on the road we ate lunch out of the cooler. We packed waters, bread, lite snacks, and sandwich stuff. Each night we'd wash and refill the water bottles and would wash the plasticware we brought. Each morning we'd fill ziploc bags half-full of ice from the hotel and that'd keep the cooler cool all day.
2. Hotels/Inns with breakfast. My kids pointed out that most lodging that has "Inn" in the name serves free breakfast. We take advantage of that every chance we get. The two nights we stayed somewhere that didn't serve breakfast we ate fruit and cereal that we kept in our cooler.
3. Search the internet ahead of time for free things to do. We found free things to do in every town. Only one day, in St. Louis, did we do two things that cost money. We also paid to go on the USS Alabama in Mobile ... everything else we did was free but still awesome!
4. We chose to eat dinners in restaurants. While we spent a little more that way we still did pretty well. One evening we went to Pizza Hut and took advantage of their $5 medium one topping pizza deal. The total bill, for 10 of us, was less than $50. I printed coupons ahead of time for restaurants and we brought those with us.