In South Dakota, my '02 Winnebago Journey Class A at 30,000 lbs would be $32/mo for the tags, $10 for the title. So nearly $400 as you said. You add in the other stuff you need to make SD your residence and you're looking at around $600/yr I guess. Most of that over the registration would be for the mail collection service.
One thing I've found is that wherever you happen to be, just google "Mail services near me" and usually something pops up. So if you really like a certain spot, you can make that your domicile, if they have that service. Right now, I'm using the US post office box as my domicile in Burns, Oregon since they don't have a mail service with a street address there yet. (I don't fit inside the PO box so I choose to live in my RV). One thing that's changed recently, is that the VA now allows us medical services recipients to use a PO Box instead of a fixed address. So that's what I'm doing in what to me is a more desirable location. You don't
have to make SD or TX your home base if you'd rather not.
Note that not all areas have mail services like what you're thinking of...they're kind of spotty right now but the industry will grow with all the boomers retiring and going out RVing for months at a time.