We have used My RV Mail (www.myrvmail.com) in Florida for several years and have been very happy with their service. We've used the address for drivers license, vehicle registration and weapons permits.
I don't know if you can register vehicles by mail initially, since there is a requirement to have the odometer reading certified (by either a Florida vehicle dealership, law enforcement officer or a DMV agent). Renewals can be done by mail.
If you contact Kirk at My RV Mail, I'm sure he could tell you.