The Texas Gulf Coast covers quite a few hundred miles, so where on the coast are you going?
If I were headed for anywhere along the central part of the coast, I would take I-35 from OKC through Ft. Worth not Dallas, to Waco, then get off onto US 77. This takes you through lots of old German-Czech farming towns like La Grange, Schulenburg, and Hallettsville with mom-and-pop bakeries and BBQ cafes for delicious breakfasts and lunches. From Victoria, you can then head south to the Port Lavaca-Rockport are or southwest to Corpus Christi-N. Padre Island.