Taking I 10 I presume.
Lots of flat, open miles. Free open land for boondocking - forget that in TX pretty much; it's not like lots of states. It has some public/forest land but I doubt you will run into anything like your thinking unless you hang around a place a while. I've given up on it and I've been here decades.
At Tucson, if you're interested, there's the Saguaro National Park; it's pretty interesting. I never knew so many birds lived INSIDE them. Tombstone is right down the road (off the interstate). No boondocking but I believe there are some RV parks nearby. It's cool to see the cemetery with all the markers of all the folks you've read about over the years (refurbished of course) - and the history is neat too. And, as you stand out there you think....WHY would ANYONE come out here to live (as I'm sweating from my hair follicles to the soles of my feet)???
Las Cruces, NM. Not much there but if you find you have to stay, visit La Posta for a great Mexican meal. Then on to El Paso. I wouldn't spend the night there, but if you're inclined to run across the border to buy some cheap Mexican curios, you could stop and do that.....just be prepared to dodge bullets and wear a bullet proof vest.
I 10 to Balmorhea is pure W TX "nothing" unless you wanted to stop and venture off into some of the little towns that border Mexico so you can see what Juarez looks like on our side - I wouldn't recommend...but they do have some good breakfast burritos in some of the places. In Balmorhea there is a park that has a really nice spring, pool etc. that folks love to go scuba diving at. It's interesting but unless you are going to use it I've found that I don't visit it much anymore.
Going on, Ft. Stockton has the giant roadrunner so may want a picture of that...other than that... If you have to stay in Sonora there are some caverns nearby, but if it were me, I would continue on to Junction.
Junction is a pretty nice little place on the Llano river. There is a park there on the river and I don't recall if you can park an rv there but it's possible. If there stop by either Cooper's BBQ or Lum's - good TX BBQ; I prefer Lum's but Cooper's is good.
Then to Fredericksburg...a great place and have visited for decades. Nice town, nice folks, pretty country and great food. It is a small, but touristy, hill country town steeped in German heritage. I suppose I've eaten in every German restaurant there and they're all pretty good; might try the Auslander or Altdorf if you stay. And, if you stay in Fredericksburg or nearby, you might want to take the time to drop by Luckenbach...as in the song, Luckenbach, TX. Changed a lot from the 70's but the history is till interesting.
Johnson City - not much there at all but LBJ's ranch is right down the road and you can check that out if you are interested. I believe Lady Bird had some gardens. Check on that.
And then.....you're in Austin! I won't go into stuff in Austin cause there's a lot so you need to look into it. I will tell you traffic is terrible. The capitol is there and it's interesting if you like those sorts of things - been in it several time for various purposes and I like it every time.
So there, my long winded thoughts on your proposed trip. Sorry that it is so long but there's a "lot of stuff out there" and I know how hard it is to try to figure out what to see and what to do. Have a good time planning your trip and a good journey.