It will take awhile to load, but you're welcome to use some of my notes for places to stop. Just place your mouse on the icon and click. I haven't updated it for awhile so there are more places to explore for you on your way down.
We head down to the RGV from the Duluth area. Getting kinda chilly up here, won't be long till there is ice in the bird bath.
Our route notes.ON EDIT:
Read what Pawatt wrote, and he has a good route (a little aggressive for a first timer, but still a good one.)
Only suggestion is do it in reverse. Coming down is when you want to do Hill Country, and SA since you got to get out of cold country quickly. Then in the spring do the Bolivar Peninsula and take your time coming up based on spring temps.
Only thing to watch out for is spring Breakers so make sure you have reservations prior. Even in central Texas during the month of March make sure you have reservations. School is out, and Texans love to camp. Once you see some of the nicer parks you'll understand why. On that topic make sure you have a site prior to Thanksgiving, another big Texan camping weekend and most of the nicer places will be booked.