Actually, any site in Glacier is "preferred" these days! We go there quite often, but the crowds have made it very difficult past couple summers to get "first come, first served" campsites . They are taking reservations at a few camp grounds in East and West now, and many folks use this service, so it is getting more difficult for "first come" campers. Even more frustrating is some of those "reservations" go empty first night, since people think since they reserved a site for X days, they don't need to show up the first day...frustrating to see that and I am told the Park system is working on a fix so they don't go empty.
Now to your question on campgrounds that allow RV's: on the West side, Fish Creek is reservation only. ST. Marys and Many Glacier on East side also accept reservations and have some spots that are on first come basis. Entering from the West, Apgar and Avalanche also allow RV camping first come bassis. On the East side, St Marys, Rising Sun, and Many Glacier. 13 Miles from West Glacier on the way to East Glacier, is Two Medicine which also allows RV camping. There are many sites of course, "outside" the park, so you are likely not going to be "homeless" should the park be full.
You can see animals and enjoy the park using any of these campsites as a base. If you boat, most all of these also offer great access to the lakes with rental craft available. Many Glacier is probably the one of the most popular for hiking and bear or moose sightings, and there is boating there also. Note that many people start out early from any campsite, and head to Logan Pass on Going to the Sun road for hiking and wildlife! Like Yellowstone, Glacier can get very busy during the day in June thru August with many hikers.