The humidity after San Antonio is the killer. I didn't find the heat so much of a problem as the humidity. I was soaking wet about an hour after I started riding.
If you are a member of AAA you can get some good state maps and plot out your own route. I recommend you avoid Hwy 190 as I found it loaded with traffic and not well maintained in many areas. You said you didn't want to spend the money on ACA maps but they provide a good route with information about potential stops along the way.