IIRC, here in Montana an RV must be less than 400 sq. ft. of living space.
A tiny house must be more than 400 sq. ft., and probably won't be legal for road use.
However, I do not know that for a fact.
Things have changed over the years. In 1950, we lived in an eight by twenty two foot M System "trailer house". NO, it was not a travel trailer, it was a TRAILER HOUSE! No holding tanks, no propane bottles, a self contained oil burner for heat, and a 20 gallon (I think) water heater. Many of the construction workers on Hungry Horse Dam lived in similar trailers. the guy my parents bought it from delivered it from Billings to Hungry Horse for them, towing it behind a half ton Chevy pickup!
Speaking of change, exactly what IS a "McMansion", and why is it called that?