Do Memorial Day weekend. It is packed, and there are people boating everywhere coming from all over the place. It is a super fun time to meet and see people from all over. Wilderness Gateway is the Main Campground, and reservations are surely booked up by now, but you might check Rec.com for something in loop D. As goodtimes says, you can always find a place to crash in the ghetto, and that is where the party is. 3 River also is very busy, but I'd expect they are booked up too, but don't know for sure. After Memorial Day, I'd say Lose it on the Lochsa. Better yet, go for Memorial Day and stay to the next weekend. There will be plenty of people to boat with, and a piece of cake to hook up with people to run with.
Living in Montana, boating in Idaho