You can purchase an 18 wheeler for private personal use and drive it as a camper. The problem is the federal law designates anything with a gross vehicle weight rating of more than 10,000 pounds as a commercial vehicle or if the vehicle is designed to seat more than 16 people including the driver. There are exceptions for private non-commercial uses. Some 3/4 ton pickup trucks and almost all 1 ton or better pickup trucks fall within this category. The state however does the licensing and can be a little loose with these definitions pertaining to cuts in highway trust money from the feds. But they don't have much wiggle room and it's easier to just include everything. This is the reasons for the pickup truck license issue and it is likely the same in most states.
What I find interesting is that many of these people, and probably most of the people upset over the laws and rules trying to be enforced, are the same people who think businesses need strict regulation and so on. Most of these people got what they wanted and are now realizing how much what they wanted sucks. People like me who hold that this excessive regulation makes it harder to competition to start and compete, that think this excessive regulation benefits not hurts the established businesses, who think less regulations but more proper and enforced or effective regulation would be the best solution, are called racist conservative libertarian kooks who know nothing. And even when those who do the calling get ensnared in their own traps, they will not admit they were wrong or even the opposing views were the slightest bit right. It's just the man putting their boots on their necks with little to no lessons learned.