Not sure I understand exactly what you are trying to do. If admittedly you run into difficulty when booking sites, how do you think it will be different with fcfs sites which are usually cheaper and likely already full? Add to that, you won't know they're full until you get there and then you're screwed. I feel like the days of loading up the camper with the romance of the open road and no plans are over. Since COVID, camping has exploded and finding sites is difficult enough with plans and booking. Can't imagine the headache of winging it especially with a TT. With a class B you'd probably have more luck urban camping, hotel lots, etc. You might try sites like dyrt, hipcamp, Harvest host, Recreation.gov, CampsiteTonight, and online forums like Reddit.