Oklahoma is no sales tax, 3 1/4% excise on motor vehicles when they are titled, vehicle registration $192 the first year registered and progressively decreasing with length of registration, four year stages for most of the decreases. Motorhomes are passenger cars in Oklahoma, if used as private vehicles.
I don't know if that is the lowest you will find, but I know that OK commercial registrations are low enough that for businesses with multi-state presence, it is a preferred state of registration, but that's commercial.
As a former resident of Michigan, however, I would not advise you, if domiciled in Michigan, to register a vehicle in another state that you would be garaging in Michigan, particularly if you have no physical presence in the other state. Michigan is one of the states that likes to be tough about collecting taxes, enough so that when I was covered by the Soldier's and Sailor's Relief Act, I still registered in Michigan and made my arguments with the other states where military duty made me resident (Florida, Indiana, South Carolina).
My late brother, resident of Florida (about 8-9 months) and Michigan, domiciled in Michigan, registered his Michigan cars in Michigan and his Florida cars in Florida. He was professionally a tax advisor, and had a pretty good idea of what he could and could not get away with, the risks and penalties. If you want to avoid Michigan taxes, talk first to a Michigan tax advisor (lawyer/CPA).