Some will say that the best place to sit in Dallas is anywhere else but Dallas, others have good luck getting out of Dallas.
A lot of drivers don't much like the truck stops around DFW so sit elsewhere. Personally, I like it up around Plano. There's a good Fry's Electronics store there, a great place called Love & War in Texas for good food and music, and the area provides easy access to the main highways, with plenty of choices for food, etc.
I have also used the Grapevine area a lot. Big shopping mall area, lots of big parking lots and good food, etc. There's another Love & War in Texas there (I'm a big live music fan and they have the best of local Texas singer-songwriters at both locations).
If you're in a van, look professional, and keep it low key, I've used hotel parking lots around Dallas a lot of times. Have even gone in and asked to use their coin laundry and wifi.