Wife and I just came thru Del Rio from SA a couple of weeks ago. Hwy 90 is my favorite drive home from SA, usually no traffic, just easy cruising, watching the landscape go by. You will be fine, most here are more irritated by the people coming from the south than the ones coming from the north. Where I'm at it's rare to hear what I call a Texan accent, seems everyone here is from somewhere else, but when I do it is most enjoyable to listen to. Enjoy your trip!
Needed to edit: 100% what Winger Ed said, small towns are the best, and don't be afraid to say hey to the guy pumping gas next to you. some of my best conversations have been there or standing around at HEB.