Hwy 2 across the USA.

Oh I love route 2 but I'm not much for freeway driving so it is perfect for us. Check out Downstream Campground near Fort Peck, It is just a few miles off the highway and a great place with a nearby interpretive center that is nice. Also a few trails and if you fish, we saw a lot of folks fishing the lake.

We stay with #2 east until Bessemer then 41 to #2 again. and south thru Mi. to Flint then east again to Port Huron to Buffalo and 90 east. our normal time is 5 days. unless we stop and fish.

It probably goes without saying, but US2 has a gap in it between the upper peninsula of Michigan and Lake Champlain. That's only of academic importance to your specific question, of course; but as a general matter, taking route 2 across the US involves either teleporting or driving partway across Canada.

Took Rt 2 all the way from the UP to Redmond, WA back in 2009, and it was a great trip. With the exception of a 10 mile stretch just inside the eastern Montana border, where they dug up the entire road to convert it to 4 lanes, and the track lay across soft sand where you took your life in your hands if you deviated off the twin ribbons pounded down by previous wagons, it was a great road all the way. I'm sure they've finished that stretch by now, but 15 days should do ya. We took 14, and made it with ease.

I see that Cranbrook is west of Glacier NP, and the Waterton Lakes NP in Canada, so of course you must see those. The Upper Peninsula is chock full of beautiful little rivers and waterfalls, you might want to take a peek at a few of those.

That's our favorite route across the US, much fewer trucks, little towns along the way to shop for supplies. much of it is 4 lane.