OP here. Thanks for the responses. Apparently there are several first come, first served NFS campgrounds outside the park. as well as commercial RV parks outside the park. That's what I was looking to find out about, as well as general info about crowds and traffic within the park. I guess I could have been more specific in my question.
I spoke with a friend who was in Y'stone last week who said the park did not seem crowded, although he had no info about RV options.
Any info about the outside the park options or general level of crowds within is still welcome.
Thanks!