I have been looking into SD domicile, including RV and Toad insurance options. I talked to a few of the agents associated with the mail forwarding companies (all nice people), but found the best deal with Heimgartner Insurance Inc is Sioux City, IA. They are licensed to write insurance in SD and a few other states. I have spoken with the owner, Travis Heimgartner, several times -- he was very helpful and his quotes were lowest when comparing the same coverage. If you are looking for a new insurance option, I recommend getting a quote from Travis. Their website is:
Insurance Company Sioux City, IA - Heimgartner Insurance Inc.