Smart idea your are asking on here for current info and "ins and outs" of membership. Over the years they have changed some of their rules. Like full time renters, which equals less spots available for traveling RV'ers. So some blogs are out of date IMO.
We've had membership over 20 years. We never stay'd 21 days. Several times during summer rush, when no spots were available most of the time they had "overflow" to park until spot becomes available. All except places near big cities. Which we don't usually stop anyways.
Sorry, We don't travel by reservations, so not up on those rules. But make sure you know all about your membership before you sign.