You might get a site in May but the Going to the Sun Road will be closed at Glacier
Yellowstone might not all be open in May too.
As to campgrounds,I think it’s a bit late now for those two places.
I made reservations for West Glacier KOA in May 2020 for July 2021 (12 days)
I guess you can check for cancellations or play musical RV sites on a daily basis. We had seen several Rvers move every day at West Glacier KOA. I think it’s very difficult to do if you had a motorhome like mine..I wouldn’t move 5 times in one trip. Just to spend 5 nights at the rates they charge.
Yellowstone is worse, less camping sites available nearby as Glacier.
My recommendation plan a year in advance for those two places.
2020 Winnebago Horizon 42Q
99 Storm 30H, 04 Southwind 32 VS, 07 Ellipse 40FD