We workamped for several years and I agree completely with halfwright. There are probably some KOA franchises that are good places to work - I know of one family-owned operation in PA that would be great if they hired outside the family and friends (they don't) - but too many are just money mills.
As for a workamper sign-up fee, many people find positions through workamper.com, which is independent of any chain. There is a subscription fee, but it is modest and well worth the price. And I can understand why KOA would have a fee to manage their own pool or workampers, if for no other reason to eliminate curiosity seekers who won't actually show up when offered a job (it happens all too often).
To my way of thinking, it is important for you to interact with the prospective employer and discuss exactly what is expected of you, i.e. your compensation, hours, days off, specific duties, obligations, etc. There is no standard workamper job or contract, no standard set of chores, and no standard compensation. If you don't ask a lot of questions, you will probably be unhappy.