Campground between Yellowstone and Glacier?

I have done this route several times. I spend the night at the Walmart in Great Falls, MT.
From Great Falls, I generally take 89 to Browning.
From Browning, you should take 464 to Babb. It is a real nice road and drive.
Then drop down 89 to St. Mary.

89 from Browning to St. Mary is not really RV friendly.

We've towed those roads from West Yellowstone to Glacier and checking Mapquest shows it to be about 9 hours. If it were us, we could do it in one day, by leaving by 6am and heading out, with plenty of stops. From West Yellowstone, take either 191 or 287 to 90 to Missoula and 93 north to Kalispell then why 2 around the southern end of the park to St. Mary. or West Yellowstone to 90 take 15 from Butte to Great Falls to Shelby then Hwy 2 west to Cut Bank and Browning which calculates at 10 hours. Again, we could do it, by trading off driving, but you may want to stop. All of those are good roads. As for a stop in between, I couldn't suggest anything since we've not stayed overnight, but you could check Good Sam's trip planner for suggesting if you're a member or campground reviews. Enjoy your trip.

From West Yellowstone, we might consider doing the drive to St. Mary in one day - US 287 to US 89 to MT 484.

From Fishing Bridge, we would drive it in two days, stopping in Great Falls overnight. - US 287 to I-15 to Great Falls. I-15 to US 2 to MT 484.

Overnight we would probably stay at Dick's RV Park; it is convenient to I-15.

Tom

My preference is to go from West Yellowstone, Quake Lake, Ennis, Boulder and then spend the night in Helena. Not a lot of choices of campgrounds but Lincoln Rd RV Park is very decent and well located and easy on/off with eating establishments and fuel & grocery nearby. Then north to Wolf Creek, Augusta and then Browning to Babb. Mostly secondary road but they are fine with great scenery. Like you we travel with a 5th wheel.

I should have mentioned that I live in Helena and if I can help feel free to PM me.

If you have extra time the state capitol in Helena gives a great tour. It's a beautiful building.