Sometimes it is best to get the help of a dealer
Start in your case by talking to the NY Vehicles folks (DMV?) They may be able to get you a "Temp" registration fairly quikcly posibbly even a downloadable document, once you buy the RV.. So might Michigan Department of State (Secretary of States's office),, alas interstate purchase is a bit above my pay grade.
Michigan does give you 3 days to get it to either A: home or B: License plate place. You must have insurance however.
A dealer can get you temp registration as well.