Going to the Sun Road is limited to vehicles 22 feet and shorter. You don't need to go on that road to get to a campground. You can take US Rt. 2 around the south end of the park.
You would have no problem getting to Fish Creek in a large motorhome. The roads are perfectly fine to the campground entrance. However you would be wise to check with a ranger who has actually worked there about the length and height. The campground roads might be your limiting point, other than the sites themselves.
Check out Glacier Meadows RV park on US 2 just outside the park. It would work well for you.
This is a link to
the campground layout. One loop is marked non-navigable to vehicles 28' and longer.