Done these routes many times. Head up 75 to York across to Sidney or take loop north to black hills then west hit highway can go west 14 to yellowstone avoid 14A unless you are adventuress. Or go north to billings depending upon desires time frames rig etc. Red lodge, Scott's Bluff is a thought. Woolrich outlet up there somewhere on the way.
Once u r in MT it is awesome a feast of choices. I live here now, 14 years in Topeka...
A bit more Going across Nebraska, get off on 26 then go 385 to the Black Hills IF you were to go that way. 385 is a nice drive. There are some nice parks including Custer State park and nice RV parks in Custer. We bought land there for retirement. That whole area is a trip unto itself but if continuing, hit 16 toward Gilette and 90 across. There is one place by Gillette where you can see the coal seams like 30 feet thick that they are mining. Interesting. Also checking out the little hillocks on that trip. They are the remains of 10s of million year old coal seam fires caused by lightning where the fires turned the coal into what are known as clinkers of hard rock that withstood weathering for millions of years leaving the hills after erosion. Continuing to minimize highway travel you can head across 14 and IF you want to try 14A, there is the "Medicine Wheel" at the top that is truly interesting. You can go through Yellowstone (without staying/since sites are already gone)and if you go through to the West Yellowstone Exit and head north toward Bozeman there are some killer National Forest campgrounds on the river about 50 miles north (some with electricity). Secret, we have found open sites on major holidays in them. You would then get back on 90 at Bozeman and Montana lays before you. From there, I'd head up toward Glacier. The trip is better than the destination IF you know where to go.
To minimize highways, head north at Big Timber right next to the Crazy Mountains (more beauty neat history still wild) and then across 12 to Helena. Nice drive, beautiful. Then at helena, continue on 12 across MacDonald Pass this road is awesome also (Rockwell Dixon Nat Forest campground at top of the pass has open sites on major holidays also and is gorgeous)and head north at Avon on 141 up to 200 and across and up the back roads next to Seely lake up to Glacier. No highways stunning drives, lots O deer though (lots/beautiful drive). The highways simply get you to the next neat place fast in Montana. As you can see, I am familiar with avoiding highways and 5ish years in MT has. Oh, that Woolrich outlet is in Alliance NE (dont know why but great deals).
So, if I had a few weeks doing your trip, that's what I would do... Oh, we do day trips to Yellowstone and Glacier... in the off Season. AND if you had lots of time, hit Kananaskis Co. in Alberta. Imagine Glacier and Banff without the people and a boatload of Grizzlies who just look at you like, what are you doin in my house. The back roads up there are unbelievable (dirt wide/cowboy trail) and only open for three or four months out of the year. Honestly I feel blessed since I left kansas... I am sure Kansan's feel cursed over the last few years based upon what I read. Oh and I gotta house in Topeka I'll sell you cheap I still can't sell after five years.