The bottom line wherever you go is the bigger your rig the more likely you will have trouble getting a site. A smaller RV will usually fit in every site in any given CG/Park. If they only have one you still get in. This is simply not the case with the larger RV's as most CG's have a limited number of large sites and the pull throughs, that so many seem to prefer or "need" are limited in number.
Being flexible as to when you are traveling helps a lot as well. As noted, arriving on Friday or even Thursday in many areas pretty much guarantee's the inn will be full. Not traveling on the weekends or holidays also works well in that you miss the worst of the traffic. We have found arriving on a Sunday or the Monday at the end of a three day weekend assures us a nice site and often with some leftover firewood thrown in. :B
If we are headed to a particularly popular spot we will often make reservations ahead of time to insure that we not only get a site but one of the nicer ones we have selected. Another common occurrence is even though a popular spot, Like Watchman in Zion, is booked up, they can have last minute cancelations or no shows. Nice sites are often available late in the day though it might just be for a night or two. The Rangers at Watchman encourage folks to check with them for last minute availability before giving up altogether. :C