Where iin the Lower 48 are TC's the most popular, least...?

Up here in Ma. I see quite a few at least 3 every trip weekends mostly,
on a nice summer weekend you can count between 25-50 on Sandyneck beach
and in the fall on Plum Island 10 to 15 on a good weekend
last year I stayed at Salisbury State Park late in the fall and there were maybe
15 sites occupied with 3 truck campers on them