Here are my recommendations from Salt Lake City north.
Logan Canyon and Bear Lake - great canyon drive and nice views - lots of places to stay for a day or two just off US 89 all the way to GTNP.
If you haven't seen Flaming Gorge, then go up US 191 through Vernal and stay a couple days around the area, then North to GTNP.
Between Yellowstone and Glacier, you have several great routes to take. Plenty of Forest Service campgrounds and private ones also. From West to East:
1. Take US 287 North out to Ennis, then MT 287 to Twin Bridges, then MT 41 to Dillon, then MT 278 to Wisdom, then over to US 93. The route to US 93 goes through ghost towns (Virginia City) and several river valleys that are excellent trout streams (Madison, Ruby, Beaverhead and Big Hole - if you're into that). Pretty country all the way. Then go North on US 93 through the Bitterroot valley to Missoula.
2. Take US 287 North to I-90, then I-90 North to either Garrison and take MT 141 and MT 200 to MT 83, or stay on I-90 to just south of Missoula and take MT 200 to MT 83, then North to Bigfork. More nice country and rivers that you see in movies.
3. Take US 287 North all the way to Choteau. It follows the Missouri River up to the Gates of the Mountains. You get great views of the eastern Rockies just the way that Lewis and Clark saw them.
You guys are seasoned travelers and will find plenty to do on this trip. I have been on all of these routes and all the roads are good for towing. There are some mountains to go over, but nothing like the Beartooth. Have a great trip...
2007 Jayco JayFlight 27.5 RKS
2014 RAM 3500 SRW CC CTD 68RFE