There is no definitive answer. Many of the laws commonly referenced apply only to "trailers". What is the legal definition of a "trailer" in YOUR jurisdiction? Here in Montana it is contained in MCA 61-1-101.
In many areas the definition clearly excludes towed motor vehicles(such as, a "trailer" does not have motive power, or is used to carry property but not people). (MCA 61-1-101 (82) (a))
However, many states have a "Braking Performance Law", where your rig must be able to stop within a certain distance (usually 40 or 45 feet) from a certain speed (usually 20 MPH) on a level, clean, dry, hard surface. Here in Montana, that law is MCA 61-9-312 (4). FMVSS also has that standard.
Many people cite ethical or moral issues, or the potential for civil lawsuits.
Since braking equipment is usually not included in any "Reciprocity Agreements", it behooves anyone who is towing a motor vehicle behind a RV to know the laws in EVERY state you enter!
I'm not sure that is practical!