It's all about attitude. With the right attitude they will have a blast. I did something very similar right out of high school with a buddy and a beater that we took out the rear seat for more room to stow our gear.
We would pull into a campground-many with no tent rules and simply stated that we were traveling the country on our college funds and needed to stay as cheaply as possible and asked for the cheapest out of the way spot to throw a tent and promised to be no problem. We were never turned away and 80% of the time we were allowed to stay for free. There were even a couple times that we received visits from the young ladies that were working in the office.
Those that were the most generous we offered to do chores like clean bathrooms etc. The generosity of the campground staff and owners was amazing and we were often offered meals and doggie bags to take with us. It was 5 weeks of my life that I wouldn't trade for anything.