Oh my gosh yes the dates for the Lake Siskiyou rally at Shasta have been set for months.
There is a post in the Western Regional category here at the MOC that should tell about it. I think it's authored by Parrothead. The rally is set for near the end of May, usually centered around Memorial Day Weekend, with some people arriving a few days earlier and some staying a few days later.
Find the post and call up there asking for a site in the "300" area and let them know you are part of the "Montana 5th Wheel" group.
Hope to see you there. Actually, we don't have a campsite yet but hope to get one even if it's in another part of the campground.
I-5 is great to travel on holiday weekends, unlike I-80 which is packed with traffic.