Yellowstone/Glacier via RV

It's been 3 years since I was in the park (Yellowstone) as a visitor, but I spent 2 summers working there. If you want to be centrally located I would suggest staying at either Canyon or Lake. The park is really just huge so you might consider staying a night or two at Canyon and then another night or two in the Old Faithful area.
The Canyon campground is very large and has lots of accommodations including laundry. From there you can do the Roosevelt and Noris areas easily and probably also Mammoth. So basically the north half of the park. It's quite a drive to Old Faithful so I'd suggest staying maybe at Madison or Grant depending on what you really want to see.
Whatever you do, make your reservations as far in advance as possible. This will give you the most flexibility because sites fill up fast there. :)

As far as flying in and renting your best options are probably Bozeman or West Yellowstone and you'll just have to check the rates from there. West would definitely be less of a drive into the park as it's right on the border. But I don't know that it's airport is serviced by as many carriers as Bozeman.

Leave a reply if you have more questions!

Hmmm, well, I never worked with the campgrounds directly, but I know you could make a room reservation a year in advance. It might be 6 months for camping. According to the website you have to call in for campground reservations.
Also, there are 2 sets of campgrounds, one by Xanterra that you can make reservations and one by the National Park Service which I believe is first come first serve on the day of!
According to the NPS website peak season is actually mid-July through August. You can still get snow in June and sometimes the Dunraven Pass road on the north east side of the figure eight road in the park isn't open due to this.
We went in August with our family 3 years ago - I made cabin reservations well in advance - and I didn't find it too busy. But perhaps I was used to it.
Since there is a limited amount of places to stay in the park I think you will find that during the week the numbers will be the same day to day, but I know more people come in for day trips on the weekends.
Have fun!