From a legal point of view, an RV is usually defined by a state's DMV (or equivalent agency) and I think based on federal DOT regulations. Usually it needs to have some minimum number of specified features, things like cooking facilities, built-in plumbing systems, an electric system, a sleeping space, etc.
If a tiny house met those criteria, it could be registered as an RV, which can have some benefits in terms of licensing, insurance, etc. If not, it would presumably just be an ordinary trailer.