We stayed at Many Glacier campground that time of year (end of July, beginning of August) and I’d be surprised if the weather would get too warm for your pet. The elevation is about 4,500 ft which keeps things a bit cooler. Running a fan and leaving some window opening should keep your rig a comfortable enough temperature for your pet especially if you can schedule your hikes for the morning. No guarantees though…

I have left my standard schnauzer tied up on the shady side of the trailer with food and water for 5-6 hours while hiking. No problem.