Just a tip - don't send it more than 4-5 days before you're checking in. That seems to be the time frame for making sure it actually gets to the person assigning sites. Earlier doesn't seem to help, although we usually have our request noted on our reservation too.
Also - remembering this is just a request and there aren't any guarantees - it seems to work best to put in your request what it is that you want/need vs specific site requests, i.e., near comfort station, dog walk, bus, etc. If you're checking in on a busy day, and limit them to just one or 2 site numbers specifically, they may not be able to accommodate you. On the other hand, if you tell them what "type" of site/loop you want, they're more likely to be able to find something that will work for you.