Ditto here - I've been a South Dakota "resident" since last June. The process is quick - whole thing took me about 3 hours and the only reason it took that long was the drivers license bureau in Madison wasn't open that day and I had to drive down to Sioux Falls to get my license. I've gone with
and am totally satisfied with their service so far. Very inexpensive - in fact I don't see how the woman running it makes any money but I guess she does. For me coming from Virginia license plates were a good bit more expensive, I had a small savings on my truck insurance but the trailer insurance was double what I was paying in VA - but that might have been because I was going fulltime. I'm with State Farm and have been for years but may do some shopping around for trailer insurance the next time I have to renew. This was more than offset by no income tax, personal property tax and the hassle of vehicle inspections.
Although I'm no expert I think that most people who choose Montana are those who purchase very expensive RVs and they want to save the sales tax. If you already have a trailer doubt if MT would be the best choice. The top 3 for fulltimers are FL, SD and TX.