All rules for using rest areas in Texas are codified as laws under the State’s Transportation Code. These are the only codified rest area rules that exist in the State of Texas. Any other rules you may see posted on signs are done on a case-by-case basis depending on the rest area itself, or by the organization contracted to maintain that rest area.
These are the actual, word-for-word text of the rules…
Title 7. Vehicles and Traffic
Subtitle C. Rules of the Road
Chapter 545. Operation and Movement of Vehicles
Subchapter A. General Provisions
Sec. 545.411. Use of Rest Area: Offense.
(a) A person commits an offense if the person remains at a rest area for longer than 24 hours or erects a tent, shelter, booth, or structure at the rest area and the person:
(1) has notice while conducting the activity that the activity is prohibited; or
(2) receives notice that the activity is prohibited but does not depart or remove the structure within eight hours after receiving notice.
(b) For purposes of this section, a person:
(1) has notice if a sign stating the prohibited activity and penalty is posted on the premises; or
(2) receives notice if a peace officer orally communicates to the person the prohibited activity and penalty for the offense.
(c) It is an exception to Subsection (a) if a nonprofit organization erects a temporary structure at a rest area to provide food services, food, or beverages to travelers and the Texas Department of Transportation:
(1) finds that the services would constitute a public service for the benefit of the traveling public; and
(2) issues a permit to the organization.
(d) In this section, “rest area” means public real property designated as a rest area, comfort station, picnic area, roadside park, or scenic overlook by the Texas Department of Transportation.