What you did wrong was not listen to the instructions that deli owner gave you. He cooked the meat at a low temperature- 150*.
You cooked it at 500* and you got shoe leather. I honestly don't know of any food I would cook at 500* unless it was in the broiler.

Cuts like London Broil and steaks cannot be used for slow cooking. You need tougher meats. Which happen to be cheaper. When you take a lean meat which is meant to cooked for minutes instead of hours, you toughen the meat. Chuck or Pot roast are better choices, but don't take my word for it. Check out the sites. Save some pennies and have fun!!

top round is so lean that it will roast like a brick.Go to a real butcher and talk to him for advice on the best choices for your budget.Marbling is critical for a good roast.A good butcher will be glad to guide you.Yes,slow cookers are for stew,pot roast,etc.
I have found that London broil,broiled and sliced correctly,is yummy served hot but loses a lot when cold
My dogs would weep if they knew you were tossing such a treat.

If I am right, for a London broil you need Top Round Roast not steaks. Then you have to marinate it in a marinade of your choice before grilling or cooking the way you want.

Some excellent answers here. We get good results with London Broil by marinating, grilling medium rare and then slicing very thin. We also like to slice the leftovers as thin as we can and put on a nice salad. To get the grilling time just right we use the cookout calculator at charcoalbob.com. It has instructions for how to grill London Broil of various thicknesses.