Definitely go and see the Flaming Gorge area, it is a very beautiful area. If you are into fishing, even better.
I have towed my TT from Cortez up to Moab, 491 to Monticello and 191 up to Moab and to I70. All that is a fine.
If I were going to Yellowstone and wanted to visit the Flaming Gorge, I would settle on this route. From Moab, take hwy 128 along the CO river. This is a great drive. Then east on I70 until Fruita. Hwy 139 north (I have not driven 139). At Rangely, hwy 64 to Dinosaur, then hwy 40 to Vernal back to 191 into Flaming Gorge area. I've driven from Rangely into Flaming Gorge a couple times and it is fine for Rvs.
Leaving FG, continue north on 191 to a short stint onto I80 (I've not driven this section), off I80 back onto 191 to Jackson (I have driven/towed this). Stay at least a few days in Jackson as it is awesome. You can visit the Tetons ad then into Yellowstone.
That would be the route I would take as it is scenic and a lot to see, minus the sections I noted that I am unfamiliar with but I would have no issues towing on that route.
