Check this link for TX parks:
You need go thru the reservation system for the most popular parks--trying to drive up and get a spot for several days usually doesn't work during the summer season. If you plan to use TX state parks a lot, the annual pass is a no-brainer.
None of the parks allow a specific site reservation, but when you go thru the res system, there will be info to input to help them put you in a space that will fit. Most of the parks I have been in have some very long pull-thrus and some very short other sites. You make a reservation, then when you get to the entrance, they will assign you a space. Most have electric and water, some have sewer, a lot have 50amp also.
You will enjoy using the TX parks....