I roasted a top round roast low and slow and it still came out dry and tough. I know that this is a cheaper cut of meat but I was wondering if I put it in the crock pot with gravy or water, could I get the roast more tender?
You may be able to get it tender, however it's still going to be dry as it has little fat in it, and what was there has been cooked out. You can try and fix it by braising it as @JoshieSimmons suggests and adding some fat to the sauce, maybe in the form of bacon.
Personally, with a top round roast I'd usually braise rather than roast it in the first place, it's much more forgiving. Plus, some meat sold as roasts has no business being given that title.
Don't leave it up to chance and vacuum marinade/brine it in the fridge with salt and bromelain(from pineapples) extract.
The salt will give the meat a juicier sensation when eaten and also slightly tenderize and cure the meat, the bromelain will slowly dissolve the meat.
Give the vacuum packed meat a squeeze every day or so until the required tenderness has been reached. And before the bromelain turns your meat into pudding, cook it sous vide for a day or so.