I don't know your Montana destinations, so making assumptions.
Going through the Midwest to NE quarter of New Mexico, then up through Yellowstone to Glacier, then back across northern Plains to Minneapolis, and down around the southern edge of the Great Lakes for the return home is about 6400 miles, about 14 8-hour driving days., leaving about two weeks to visit places. I don't see this as leisurely (pace of about 100-300 miles a day for me) but it is not either terribly rushed.
Going through Duluth, the U.P. of Michigan, then down through Michigan to Toledo, adds about 200 miles, and a day to the travel time.
Crossing into Canada at Sault Sainte Marie and going across Ontario and Quebec to New England shortens the driving distance slightly but adds a day of driving.
Campgrounds that are closed for the winter start opening again beginning of April in the mid-South and the southern part of the plains (Missouri, Oklahoma, northern Texas, New Mexico. As you go further north, you may find opening dates in May, and I've found some opening as late as the weekend before Memorial Day in the northern part of the Midwest.
