Bob,
I have been using Bennett Law Office, P.C. as my Montana agent for 15 years. As a fulltimer, I have had no issues with his service in registering 2 coaches and 4 cars over this time. However, if you are not a fulltimer, there are several states, especially California, that aggressively challenge any Montana licensed vehicle that may have an owner as resident in that state.
There are several perks to using Montana registration for your coach. The most obvious is no sales tax, but another one is that any coach over ten years old, and that is all Beavers, get a permanent license plate sticker, so no more yearly license plate fees. This savings alone will more than offset the yearly LLC maintenance fee to your lawyer.
Gerald