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

You don't see many of them in the Southeast. This is the land where the 1/2 ton pickup reigns as a daily driver for LOTS of households. So when they start looking at RVing, the easiest route in is with something that will work with a 1/2 ton, which eliminates 90% or more of truck campers.

Most people don't have any use for a 1 ton pickup, so buying a dedicate expensive vehicle and relatively expensive truck camper just doesn't make sense when you already have a 1/2 ton pickup. That's exactly why I don't have one.