Starting in the late '50s on up to the late '70s truck campers were the most seen in the OK/KS/MO/AR/TX/NM/CO areas around camp sites/big COE lakes/state campgrounds.
In OK we had four different truck camper mfg that sold mostly in the south and in mid USA.
I live on the mother road (old US-66). We do see more TC's/4x4 truck on the old road just about all with out of state tags. Most truck campers around here are on a one ton DRW truck pulling a bumper pull horse trailer (rodeo/horse show folks).
We started with TC's in the '60s and '70s era and I would love to get back to one. Now if the wife could give up her 28RK house on wheels......... my life sure would be simpler