80 miles a day on tour, with 30-50 lbs of gear, is pretty ambitious. Fixed should be fine since the route is fairly flat. But it's going to be more like 450 if I'm measuring properly. I'd either start further south or allot more time.
Re: water, you want 750ml per hour of riding. Possibly more since it's gonna be hot in June. More critically, you need electrolytes and calories (~250 per hour). I'd get some powdered Gatorade, so you will get your salt, potassium and energy. A "dromedary bag" may be in order.
I'm sure you know TX better than I do, but it does seem that there are towns about 10 miles apart most of the way, 20 max? There should be restaurants, truck stops and so forth on a regular basis.
If you don't feel like investing in a portable GPS, you can spend some time with Google Maps figuring out smaller roads and the locations of restaurants, hotels and campsites along the way.
Camping or a tent is optional, depending on your comfort level. You could stay in hotels if you have the budget; otherwise get a tent (5 lbs or less). You could use a bivy sack as well, if weight is really an issue and you're comfortable with that. I'd also use a sleeping pad.
I wouldn't stealth camp in TX -- or to be more precise, don't camp on posted or fenced-off property, trespassing laws are pretty strict there. Ask permission before setting up a campsite on someone's property.
Bike Tools:
- multi-tool
- tire levers
- pump
- spare inner tube (at least 1)
- patch kit
- chain lube