I have stayed at Hobson once and driven through Faria once. I was lucky and got a spot at Hobson just by driving through. I haven't looked in a while, but when I went there were no reservations - first come first serve. It was nice as they have full hookups in some of the spots. Both Hobson and Faria are pretty tight to get into. I don't know what the limit is. I have a 35ft travel trailer (at the time of my stay at Hobson, I had a 32ft class C), and there were probably only a few spots that I could get into. I would highly suggest parking on the road and walking down into the campgrounds before driving in, no matter what you own. When I stayed, I know you can either leave cash in a envelope or login to the website posted on the sign near the envelopes (I don't remember the website address). Things might have changed and they may take reservations now, I can't be sure as I haven't looked in a while.
If your going to the area, I personally like Rincon Parkway better than both Faria or Hobson. It is located between Faria and Hobson. It is parallel to the beach parking on the 1, so you do get traffic driving by, but I have stayed a few times and it doesn't bother me (I am a really light sleeper). The views from Rincon are so much better than Hobson or Faria in my opinion. There is more room for each campsite as well (in most cases, there may be some spots that are closer to the sea wall). Most people will be parking parallel to the sea wall, don't be afraid to park diagonally. There are no rules, just have to be within the lines. If I can find one, I will usually park at the most north spots as it is easier to get down to the sand/water.
As far as finding a spot at any of these, like I said before, I think you just have to show up. Good luck.