If I opt to register in MT or another state it would be done within the laws of any state(s) involved it is not to per se evasion.
As an example in FL if the RV is removed from Florida within 45 days after purchase and remains out of the state for a minimum of 180 days, the vehicle will qualify for the partial sales tax exemption, and discretionary sales surtax will not apply despite the residency of owners, stockholders, or partners of the purchasing entity. The purchaser should keep documentation to prove the partial exemption.
Or if purchased outside FL An RV purchased and used in another state for six months or more is generally exempt when brought
into Florida, provided:
1. The owner has owned it for six months or more.
2. The owner has used the RV in another state or states for six continuous months or more under conditions that would make it subject to tax in another state.
If those rules are followed and fit your use it is legal.