Hey OP, sorry to hear you are in a bad spot in life right now. I didn't read all the posts in this thread but I didn't see anyone reference this Wiki link yet: http://sfhomeless.wikia.com/wiki/San_Francisco_Homeless_Resource Go read through that in case you've not done so already.
Personally, given your situation and stated intentions I'd recommend for you to, depending on your skill set, register a profile on all the online freelancer portals you can (oDesk / Elance are just two, Google will get you more) and to start bidding for work there. Unless you have a sought after skill the pay is not great, but it beats not generating any income with your time and working only 18 hours a week is not going to net you enough cash to stay afloat, especially in SF.
Furthermore, try to figure out a plan, for the immediate future, for commuting between whatever place you find to spend the night at, a local library which should get you internet access and power for your laptop and your 4 day 18 hour job. Any time that is not spent on meeting your hygiene, food, sleep and other needs try to do work online to get more cash. Include in this plan how much you need for your transportation pass which is likely your most important asset and pay for that as soon as you have the requisite cash saved up, where you can get food for free, a conveniently located laundromat, where you can find a place to get cleaned up yourself and what other items that you may need to pay for (e.g. luggage locker for your bags if such a thing is available in SF?).
If you find that you do not have a lot of skills to do freelance work try to expand your skill horizon. There are literally hundreds of tutorial oriented channels that can teach you a marketable skill on Youtube and other material available online for free if you have even a yellow belt in Google-Fu. I realize this is not a fix for the immediate future but you need to think mid to long term as well.
Finally, while your post indicated that you do not want to go back to Texas realize that there are a lot of other states you can get to relatively inexpensively with a Greyhound ticket and which are a hell of a lot cheaper than SF, in some instances offer much better job prospects for entry level labor and don't have those idiotic hills that make moving around on foot a punishment in and of itself.