Honestly, there are very few hole in the wall type places other than what the previous posters have mentioned. Rockwall is mainly a chain restaurant town. Most of the non-commercial places are located around the downtown area. If you feel like driving a little there is Big Daddy's BBQ north of town at 205 & 78. It was listed in TX Monthly's top 50.
A true hole in the wall route place is Boot's Burgers. It's run out the guy's garage and I think only open on Saturday until they run out, best burgers in town.