By the way, how long of a drive is it to the gulf from the SA area, and what is the speed limit on the interstates?
Corpus is about 144 miles/2 hours from San Antonio, and speed limits are 70 MPH but you'll find most folks going 10+ over that. Just make sure there's always someone going faster than you are and you should be fine!
Also note that on summer weekends, that time estimate will go up due to an increase in traffic.