When you used singular for "a one bedroom in say, Las Vegas", you appear to be thinking that it is one resort with various ways, RCI, Marriott, Hilton, to book that room. This is entirely wrong. IF Marriott has a property in Las Vegas, which they do, it is Marriott's Grand Chateau, you need to either own a week there, or somewhere else to trade into that resort via a 3rd party exchange company, Interval International, or MVC Abound/Destination Club points and the number of points vary by time of year and you book using their internal system. Then there are different inventory pools.
If Hilton has a resort in Las Vegas, which they have at least 2, the same analogy applies.
RCI is a third party exchange company and you can use a week or RCI points to exchange into a Las Vegas property that is available in their system.