Find a state farm agent who has done these before. The imperitive part is to actually meet with a local agent and not call some 800 number. These policies have to be hand-walked through the underwriting process. They'll probably write a slightly more expensive "personal use of a commerical vehicle" policy until your conversion is complete, then convert you to an RV policy.
Have patience, it can take a few weeks to even get an approval, and a few more weeks after that before you have ID cards.
My agent can do IA, IL, and WI and might be able to refer you to someone in MN...