We have worked as KOA Workampers, but it's been a decade since our last stint with a KOA.
Every job is different in terms of hours and what they consider compensation (site, pay, partial site, etc.) because each KOA is independently owned. Not all offer a free site. Some just offer a discount. Some expect you to work 40 hours/week for only a free site. Others have a lower work requirement. There are other, free, options for finding workamping jobs. Workamper News was one we also followed.
If you plan on staying at any KOAs in your travels, then get the membership. If you stay at a KOA more than about 8 nights, then it will pay for itself. Then you can check the listings, and staying there speak to people at KOAs and get a feel for how they like working there, their duties, and experience required. Since they are franchises, it will vary according to what the owner/operator wants.
We have frequented many many a KOA in the US and Canada. Most have Work Kamper's as campground hosts, maintenance staff, and foodservice staff. Without exception most seem to enjoy their roles as they are extremely friendly and go out of their way to be helpful.