Can't say I've tried them (fish tacos just don't sound right to me) but I know they're on the menu down here. Hula Hut on Town Lake in Austin is always a cool place to take newcomers ... and I'm pretty sure they have fish tacos.
Sorry ... no choice places that I know of to eat on your route. It's always fun to check out local places. I'd suggest pulling off the hwy and picking a place or asking a local.
If you get to Lampasas, Burnet, Kingsland or Marble Falls ... there's this greasy burger chain called Storm's ... old timey greasy burgers and fries.
Mac's on IH20 in Arlington has the best breakfast buffet ... the strawberries and pineapple are to die for. And I hear their other meals are exceptional as well. The Mongolian Grill is another place I loved in Arlington.
Have a safe trip!
nancy
