"Must sees" and other info can be found in the FAQs at this link:
http://glacierparkchat.com/phpBB3/viewt ... ?f=1&t=580
Otherwise, I don't know the answer to your question about RV length at the Many Glacier campground (it is located in one of the park's most scenic locations, however). I do know there's a 25-foot length restriction on the Going-to-the-Sun Road, so you won't be able to travel that in your RV. I suggest you call the park office directly at (406) 888-7800 to see if they can help with this particular question.
Also, note that Apgar is on the opposite side of the park (west side) from Many Glacier (east side) and will require a 2-3 hour drive each way between the two areas. And you'll have to use Highway 2 along the southern border of the park if you plan to go back and forth in your RV.
If your goal is to spend most of your time in the Many Glacier region, you won't want to make Apgar your base.