The Montana and South Dakota registrations can be done legally, but usually to be legal the motorhome needs to spend "most" of it's time out of your home state. And if just being stored, it likely needs to be stored where it is registered. So if you live in California and you buy a motorhome that you keep in Montana it can be perfectly legal to register it in Montana and in fact it is probably required my Montana law that it be registered in Montana is that is where it's going to be stored and mostly used. And you can even bring it back to California, but since you live in California there are strict time limits as to how long it can be in the State. I don't know those time limits but it's not a lot of time, like 30 or 60 days. The problem is nobody ever does this when they do the out of state registration. They just bring them right back to California and keep them here.
