The only way you are going to do a road trip for cheap is to know how much money you should be spending...and then don't spend more than that!
Use an app or good-old paper and pencil and keep track of your daily expenses.
Bringing as much of your own food as you can will save you a lot of money on meals and drinks.
Camping is a very affordable way to find cheap accommodations just about anywhere you go.
By getting your vehicle checked over before leaving, you can avoid some common problems like a dead battery or low fluids that can cause issues on your trip and cost you money.
You can save hundreds of dollars on accommodations, attractions, and even food by traveling outside of the busy season of your destination.