The regions North of the Yukon River, all the way to the Arctic, are the worst for mosquitos in general. The worst time of year to go is in July, however you are likely to encounter them as soon temperatures remain above freezing for a couple of weeks.
You can bring repellant, but the stuff that repels the best is bad for your skin and clothes, and will contain a measured amount of deet. Since I personally prefer to not to crop dust myself, you can purchase netting to go around your hat or head, and maintain long sleeves and pants throughout the day.
If you still get bit, don't scratch!
Yellowstone National Park lodging concessioner Xanterra Parks & Resorts operates gift shops in hotels throughout the park. Bear Den Gift Store At Old Faithful Snow Lodge The winter-themed Bear Den Gift Shop is located in the Old Faithful Snow Lodge, and offers the largest selection of youth-oriented products in the park. Bear ...READ MORE
With 12 locations throughout Yellowstone National Park, each located near major attractions, Yellowstone General Stores are convenient places to rest, refuel and enjoy one-of-a-kind Yellowstone shopping to find something unique to bring home. Each location offers a selection of groceries, camping supplies, souvenirs and Native American-inspired collectibles. Old Faithful General ...READ MORE
From what I can tell from the NPS website, many of the RV spots at Glacier are designed for 26ft vehicles or smaller, while others accomodate rigs up to 32ft.
Full info on all the campgrounds, including seasonal opening and closing dates, is available at the NPS website:
Click the tent icons on the map, or the name of the facility you're interested in on the list below the map for detailed info, and contact information for the park.