It's just one of those things that's gonna take lots of time and luck. Some people get them right away, some people can hunt for weeks before finding one. I knew of at least one person who was level 24 before they managed to get a skull to drop...and it wasn't for a lack of trying.
My experience...
I'm not going to get into a discussion about how Treasure Hunter works. Truth is, I levelled my thief to 30 and I'm still not sure, but I do know that on one occasion, we took an alliance (2 lvl 20 thieves in one group, a lvl 45 thief in the other) in Gusgen mines...we got 4 skulls in the span of about 2 hours. It was probably luck, but maybe not. With the time that could be invested in getting a skull, it's not a waste of time to try it out for yourself.
If you can't get into Gusgen Mines, go to Valkurm Dunes and find yourself a ranger. A good spot for your group to sit is at the Outpost, and have the ranger run all over the zone scanning for ghouls...if they know how to pull, they should be able to bring them back to camp from anywhere in the zone without getting hit. You'll kill far more ghouls in one night than most groups do, and the consensus on my server seems to be that ghouls in the dunes drop skulls more frequently than the ones in Gusgen. Personally, I have found at most 6 ghouls in one night as a ranger, which is far more than I found when I was hunting for my skull without a ranger in the group. And bring a thief, if nothing else, for good luck. =D