I arrived in Grants Pass on a Saturday around 5:30PM in early August last year. I aced out someone who was 30 seconds behind me to take the last spot. The person behind me had to wait 20 minutes until someone moved. When I departed around 6:15, there were two vehicles charging including me.
I arrived at Mt. Shasta on a Monday afternoon in early May this year around 12:30. All four spots were taken. When I pulled up, a diner sitting outside flagged me down and said that she had enough charge, and she promptly moved her car so I could charge. Ten minutes later another driver arrived and had to wait. By 1:30 (I needed a range charge) my car was the only one left.
While I defer to your more frequent travels and experiences, my opinion is that a lot of the congestion has to do with the time of day. Grants Pass is about 250 miles from Portland. Mt. Shasta City is 220 miles from Sac. I think it is reasonable to assume that travelers depart within narrow times in the morning and arrive within narrow times in the evenings or nights. And charging being one of those activities where we like to kill the two birds with the one proverbial stone, we will see more congestion at sites that have decent dining facilities during popular meal times.