You only pay 3% on the difference between the 2 vehicle purchase price if you trade vehicles. This is a substantial difference that how other States handle sales tax. The license and registration fees and the insurance rates are substantially higher than FL (or previous State of record) and need to be factored in. If you have no plans to buy a new coach I would not recommend moving as you will get screwed like we did. I needed a residence address and did not know that I could have done the same thing in FL that I did in SD. My FL tags cost less than $100 and my SD tags were close to $600. I have to buy a new RV soon to make it all worthwhile.
The mail forwarding services will not give you the "true" costs of moving your residency to SD. The only good that came from this is jury duty. I got in a bit of trouble (resolved) when I missed jury duty (while in CA) but SD makes it pretty easy to deal with.
If I knew then what I know today, I would have my vehicles registered in FL.