www.freecampgrounds.com , Passport America book, Fair Grounds
Keep your eyes pealed for city and county CG's some are free.
Trailer Life or Woodalls CG guides. They also show some state and federal CG's
I am a full timer.
Twice in four years, for a holiday only, did I make any reservations. I was turned away from a campground four times in that time but the CG called around for me and sent me elsewhere which turned out fine.
You will not encounter low bridge problems on interstates, US highways or state roads. I have, shall we say, investigated alternative routes ( says I to my dog "Snickers I goofed again") and I have never found a bridge too low to pass under. A few were a bit closer than I would prefer and I stopped at it to see about clearance. Then I went under very slowly. I am about 12' 6" tall. If a big truck went under I am good to go there too.