The first year I full timed, I did not make reservations because I wanted to be "free" to change my mind. And at times, I could not get into the park I wanted to at the time I wanted to.
Now I plan my travel (part of the fun of RVing) and make reservation up to 6 months in advance. I can still change my mind but I have a plan.
If you are flexible and can dry camp for a few days (this is why you have a self contain RV -- right) there are a number of BLM/FS campground near Yellowstone and Tetons some even take reservations. Others are first come, if one is full the ranger can point you to one that may be open. You may need to drive a few more miles, but that is the cost of not making reservations. Check out http://www.rvparkreviews.com/ and the boondocking section of RV.Net
