Well, the plot thickens. Just hung up the phone from the SD Dept of Motor Vehicles - for those not familiar with SD it turns out the SD vehicle license fees, while not based on value, are not insignificant. I currently pay about $560/yr in AZ, most of which is a use/personal tax. While SD doesn't have that tax, they do have a weight fee. In my case the weight fee will be about $430/yr, so I'll save a little over a hundred per year, but not the $500 I thought I'd be saving. For those with rigs worth a lot more than mine and who currently reside in high personal/use tax states, the savings could still be significant.
I've attached the SD license fee chart. SD assigns a renewal date to plate holders based on the first letter of the last name then, based on the initial registration date, the first year you pay a pro rata amount in months to get you to your "anniversary" month. In subsequent years you pay the 12 month amount.