NJ to MI

If coming through Ohio via I-80 I'd suggest that you catch US23 near Toledo and take that north through Ann Arbor. US23 hooks up with I-75 just south of Flint. Take that north to the U.P.

Not sure where you're referring to "few gas stations and no camping"? You wouldn't be talking about Michigan because there are plenty of both all along the route I suggested. Use the website below to research campgrounds on whichever route you decide.

http://www.rvparkreviews.com/

If coming through Canada I'd enter Michigan at the Sarnia Ontario/Port Huron, MI bridge. We've taken this route from Michigan to upstate New York to visit the Adirondacks. This route took us though Toronto and is pretty direct and about the quickest route. You can always opt for the route east and north of Lake Huron, Hwy 17. I've never taken that route, but it looks interesting. You can enter Michigan at Sault Ste Marie. I've crossed there a couple of times and found it very easy, but then again, all the bridge crossings were easy. I've found that it's easier crossing from the U.S. into Canada rather than entering the U.S. We've been hassled coming back into the U.S., both in Sault Ste Marie, MI and Ogdensberg, NY.

I've lived in the U.P., Marquette specifically. Spring weather can be a bit chilly, depending upon what part of spring you're coming. Early spring will mean snow....on the ground and in the air. After mid-May to mid-June would be your best time frame and even that can be on the cool side.

Thank you for the solid info.

Another option would be off I80 in OH and then through lower MI up to the UP. What is the best route? There seems to be sections of few gas stations and no camping.

Cabin,

You pretty much can't get where you want to go with crossing at least two bridges. I have crossed them all and the only that is a problem is the Mackinac. There, they might hold you up until the storm passes. Words about going through Canada...
Have passports for reentering the US.
Buy fuel before you cross.
Don't try to transport any perishable foods into the US even if they were purchased here.
No Firearms, pepper spray or other such.
Be ready to have the TT searched at Port Huron. That can be anything from casual to PITA search.

I can't even begin to advise campgrounds except the two that are east of the locks at Saute Saint Marie. Both are nice. Plan a full day there at least. Without knowing your daily range, there is not much that can be said.

There are state campgrounds all over Michigan's northern half of the lower and all of the upper.

As to what is the earliest, how good is your heater?
You do know that in the UP (Upper Peninsula), they don't even use the snow until it is a year old? (that is a joke) We have never had a choice, but were always event driven. If you look up the weather data for the major settlements, you may be able to determine when temperatures get to be more to your liking.

Safe Travel

Matt

Thank you for the suggestion. I am not sure of where I should cross, Buffalo might appear the fastest, but I do not look forward to those bridges. Are there any reasonable alternatives?

Where do you want to cross into Canada?

If you do not mind driving through Toronto may I suggest 81N to Canada.
Many state parks in the Thousand Islands of New York including Grass Point state Park opens Memorial Day or Wellesley Island State park open all year and right on the water close to 81 and 5 minutes from the customs station.
I would cross back into U.S. at the Blue Water bridge off the 402 in Canada but there can be very long waits at U.S. customs to cross not unusual to have a 1-2 hour wait