I just stayed 2 nights @ Mustang Island State Park. The showers/restrooms need lots of work. Paint peeling everywhere... Texas Parks & Wildlife now charges an entry fee for persons 13 & over. I don't think they're using the new fee increase for any improvements.
The campsite was nice - just be prepared to hike 1/4 mile to the restrooms (sure would be nice to have a port-a-potty on the other end of parking lot). The guest host trailers were conveniently located next to the bathrooms. On the beach there was a day use area with bathrooms, but that was in worse condition (mold, portions of building look like it had been burned). The beach is clean, but I think more work could be done. I don't think I would stay here again unless I had fully self-contained trailer.

Spent Dec 26th - Jan 6th at Mustang Island State Park. Intended to stay 2 nights and move on to other parts of the state. Found the RV Park, the beach, and the weather so nice, stayed here for entire time. Great beachcombing weather and very clean beaches. Clean & spacious, RV sites, helpful people at the office. All around great place to and relax with a good book. Just another example of our exceptional state park facilities in Texas. Padre Island National Seashore close by and also a great place for relaxing beach combing.