Have you considered flying to Calgary and renting there. They have a much better selection of rentals and I've found better prices. We are planning to do this next May. The airfare for us appears cheaper to Calgary than any place in Montana. It isn't that far to drive from Calgary to Glacier. If you are interested in taking a look in that direction, I'll come back and post links to camper companies and booking companies where I've found good deals. We rented in Calgary once before for a trip to the Rockies and rented from CanaDream. I have no complaints, but I'm finding a better rate elsewhere so probably won't go with them this time.
It also depends on how many people you have. We are only two people, and the campers at those places in Bozeman are much larger than we need. We rented once from Cruise America in Salt Lake City. The smallest they said they had then was 22 feet, but they gave us a 25-footer, which was very cold because it was hard to heat.