I know the OP is staying with South Dakota, but in case this thread interests someone else...
I ran across a mail forwarding service in Wichita Falls, Texas. I wonder, though, if it's good for RVers because of its strict forwarding schedule--you sign up for weekly, biweekly, or monthly forwarding, and those happen on fixed days. For example, if you're monthly, the mail is sent on the first Thursday of the month. No way that would work for us because our travel isn't particularly regimented. In fact, we usually let the mail pile up until we're confident where we'll be 5 days hence, and that doesn't happen often.
But if someone workamps or stays somewhere for the season or travels from RV park to RV park on a little bit of schedule, it might work.
https://www.texashomebase.com/mail-forwarding/