Are cameras allowed in public restrooms in Texas?

You can take a camera anywhere it is not specifically restricted by law or posted on private property (like a concert or sport event).

Taking photos is another matter as you may be in violation of people privacy to photograph them in a restroom where they can expect a certain level of privacy. Photographing people in this situation especially without their knowledge may warrant arrest and prosecution just as peeking in a bedroom window. You may also be subject to civil penalties (getting sued) or physical attack. Criminal penalties may be at the discretion of local prosecutors, judges and/or juries and could well result in a sex offender label to explain to your family, neighbors, etc.

You can use models and shoot on private property if you have the owners permission as well as all of the subjects. This process may artificially simulate an invasion of privacy for purposes of art or commerce.