Glacier is a looooong day (or a couple shorter ones) from Yellowstone, and well worth the time if you have it. If you only have a few extra days, and need to be heading back home, the Black Hills are a good suggestion.
FYI, Bannack SP (Montana) is dry camping only, but there is a decent FHU private CG in Dillon - Southside RV Park - and the owner just loves to help customers with sightseeing options in the area.
If you do want to consider COlorado, but skip Estes, some of the CO SPs are really nice. They are not FHU, but some have electric hookups. We've been to Mueller SP (west of Pikes Peak) and Golden Gate and both are very nice.
Lots of great choices in the Black Hills - if you take I-90 back from Yellowstone, you can visit Little Bighorn National Battlefield (worth a day; there are a couple decent private CGs nearby) and Devils Tower - the KOA there is right at the base of the tower, great view! Lots of good CG choices in the Black Hills and plenty to do, not just Mt. Rushmore.