With the liquid from the toms plus a small swish of water to wash out the remaining tomato stuck to the sides of the tin it was fine for the slow cooker, because there's really no evaporation from a slow cooker. The beef renders out fat as it cooks which helps loosen the mix up as well.
If I were to try to cook it on the hob for 2 hrs then even on the lowest heat I'm sure it would need some liquid adding from time to time to loosen it up.