Should a roast be covered in liquid in a slow-cooker?
It doesn't matter if it is covered or not. The inside of the slow cooker will be warm enough to cook the meat. Braising (not submerged) and simmering (submerged) are two methods which both can lead to good results. 

The "very chewy" result sounds like choosing the wrong type of meat for slow cooking. If it was a real roast, then this is the obvious problem. Roasts should be roasted, not slow cooked. If it was an actual slow cooking cut sold under a bad label (these things happen), then probably it was cheap meat from an animal grown up without movement and slaughtered too young. You have to choose tough meat with lots of fascia streaking for slow cooking.