They are located in the RV section of the park.
The showers are in the RV section as stated above. There are two showers in either the men's or ladies bathroom with one each being ADA compliant. These bathrooms are also slightly bigger than the other set of bathrooms.
https://www.google.com/url?sa=t&source=web&rct=j&url=https://tpwd.texas.gov/publications/pwdpubs/media/park_maps/pwd_mp_p4505_049h.pdf&ved=2ahUKEwizub2gwcTsAhVJXKwKHbdzBI0QFjAAegQIOhAC&usg=AOvVaw3HxkoULemnqwD7YSI9oxgc Appears to be only 1 shower where the San Marcos trail head. Bottom right loop on the map above
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