My eye of round roast has been in the slow cooker on low for a little over 5 hours, is still tough. Will continued cooking make it more tender?

For any cheaper cuts of meat, and actually expensive ones the way to go is Sous Vide! They have really come down in price and your food will taste like restaurant quality!! Google it!

I’ve never found an eye of the round particularly tender, regardless of how it’s cooked. Nor tasty, for that matter. Next time, try a different cut of meat.

I can't really give much advice about slow cookers, but eye of round, while it braises well, does have the grain running in the long direction- as opposed to the usual short grained pot roasts- so it can be expected to be stringier than what you may be used to.