The R20 leves can net you some skill points but they will never replace being in a party. There are a number of camps available out there that many people are using although I think many are being kept a secret by the community because people dont want them overcamped - annoying but you can understand it I guess - When I do the R20 leves (I am 21 now) I do solo when I have little time and group up when I have more. Provided the members of your groups arent too high this can get you more SP especially if the mages dont nuke and the mele dont overkill on high damage moves. Its all about hit the mob as many times as you can (and as a mage curing as much as possible) to get the most SP. My average depends on if I'm solo or in a party and what level I am. But from about 14 or whenever you can do them, I always solo 1 star but use guardians aspect to try and maximize sp. Now I am higher and close to being able to do the R30 leves I tend to do them all very quickly for gil/drops though. The problem is that because SP is sort of random some leves you can get well over 1000 and other you might be lucky to get 50! When you have done the same leves a few times you will see that ones that involve humes, dodo's or crabs tend to give you more SP because the mobs are more difficult to kill/have higher hp etc. Just increasing star rating means that many of the mobs just hit harder - although this is not always the case.
But back to my original point, partying will net you way more SP. Try going over to bloodshore, shouting for a mele tank and a couple of other players and go do some crabs. I was getting an average of 300-400 per kill there when I was about R16 (I know you are lower atm) - honest I was amazed at how well it worked. A few hours grind and you will rank up real quick.