Yep--I'm another frozen roast into the crockpot person. I chop onions,celery, carrot and garlic for the bottom. (Here's another hint that works well--puree the chopped veggies before you put them in the crockpot and they become part of the gravy. This is especially good if you take the cooked meat out and cut into pieces like stew, return it to the pot with par-boiled potatoes and any other veg you like)
As far as the tough roast--was it completely sealed in its cooking pan? That's the only reason I can come up with.