As mentioned, unless you plan to change your residency from MI, there is no way out of paying the 6% sales tax. When you go to register/plate the RV, they will charge you any sales tax for which you don't already show proof of payment. Yes, we have permanent trailer plates here in MI, but you must purchase the permanent plates each time you purchase a trailer. And you must pay sales tax on the full sales price of the RV, regardless of your trade-in value. One thing going on temporarily to save a whopping $150, is they will give you a sales tax credit on up to $2,500 of trade-in value when purchasing from a dealer ($2,500 x 6% = $150 sales tax credit). This should already be figured into the sales tax calculated by the dealer.
see this link