We've not found it difficult to get into Escapee's Rainbow Parks, but we don't travel to high season tourist areas if possible. Sumpter Oaks in Bushnell, FL took a couple days adjustment to our schedule to fit in during Feb a few years ago. We don't got to Racoon Valley or Turkey Creek in the summer season.
The co-ops are a completely separate deal. They are owned by the co-op members, and many don't want to be bothered by reservations. If you stay at one, you are literally borrowing another RV's personal property/ spot. Even if it is an unsold spot, or a community owned spot for visitors. We've stayed at barely improved spots, and in very fancy improved spots with a casita and patio. We are very respectful of the personal space we are allowed to stay in.
Of course the three in Arizona are crowded, maybe full in the winter. Park Sierra near Yosemite is busy in the summer. I'd expect their busy season starts when the snow melts. I've heard the two in Oregon and Washington get full in the summer also.
As far as discounts at regular parks. We've stayed in two, and both were a surprise. When we stay at a park with no discounts, we always ask as we check in, usually about PA. Twice we've gotten discounts with SKP. One only offered discounts to SKP, the other did several programs, and SKP was the best. We've also stopped at one which was in the SKP directory, and were told that a new owner decided to not offer that any longer.
You sound like you will enjoy being an SKP member.
There is an active boondockers group within Escapees, and there is always an almost week long SKP event at Quartzsite each January.