We have been through this at work over the last few years. Every state has different laws when it comes to plates, etc. but pretty much anything with a GROSS COMBINED VEHICLE WEIGHT aka truck and trailer (i.e. any newer truck pulling a car on a any trailer) must have a log book, DOT #, record of inspection, etc, etc, etc. Does this apply if you are a recreational vehicle? Depends on the state. For instance I do not need any tags, etc. here in Oregon but if I tow to CA there better be a tag on the trailer or I risk getting messed with by the law.

That is simply the tip of the iceberg. I'm going to name my "street car" '66 Charger NOT FOR HIRE if I ever get it out of storage again. Don;t ask why it's in storage!