In the same parking lot as the Toys R Us as the Mariscos El Pescador truck, there's a truck doing what I'm told are authentic tortas ahogadas.
I went and tried one yesterday. Unfortunately, the only option is pork so I wound up getting one with beans only, which basically amounted to a roll with a scant smear of bean on the inside. They graciously charged $5 instead of $6 - I imagine that the pork is probably more than $1, but if not then it's probably not the greatest meat.
The sauce was fantastic though, intense chile de arbol flavor. I ordered mine hot, but was detecting no heat. I asked them if they made it hot like I had asked, and they told me they made it medium. Pretty annoying, and a big pet peeve of mine. They did apply the hot sauce to the second half of my sandwich, and I have to say it was brutally spicy. Probably on par with a 7 or 8 at Sab E Lee.
Overall, while I enjoyed the bread and the flavor of the sauce, I doubt I'd go back here. If there was a better vegetarian option, I'd consider it, but $5 for a roll w/ 2 tbsp. of beans is not my idea of value. I guess it's not a coincidence that the line at the mariscos truck is far longer.