you're right, tiny houses get past all of the same zoning laws as RVs.
I'm not a proponent of tiny homes or RVs. our "hotel on wheels" is a giant used van we bought with 150,000 miles on it. we pulled out some back seats and blow up an air mattress at night. No bathroom, double burner propane stove with folding table, and plenty of storage space for our bikes, surfboards, skateboards...
If I had to put my finger on the difference, I think that the tiny home is the "trendy" young person way of being different and choosing an alternative low impact lifestyle that accomplishes some of the same things that an RV does. But RVs have the stigma of older retired people who are wasteful (I'm not saying it's true).