When you call or fax your request in, you will need to do it 4 days out. They do not assign the campsites until then.
We usually fax and also call and they have always been very nice. I have always gotten the loop I have requested. I may not get the particular site that I requested, but they do there best to honor it.
When you make your ressie to begin with, tell them your requests and they will put it on the ressie. Always put your most IMPORTANT request first. Because if you have a long list of them, they can't honor them all. But they will try to at least honor the first couple of them.