A crock of shit is literally another name for a chamber pot — that large jug one used during the night in one’s bedchamber when one did not have an indoor lavatory.
The “crock” part is from language as old as we can fathom form all over Europe meaning pottery — hence crockery for plates and bowls, or anything made from clay. A potter potters about making pottery; but he used to be called a crocwyrhta, or crock-wright.
To call something a “crock” means it is no good, useless — probably associated with the bedchamber use.
A crock-pot is merely a pot made of pottery in which something is cooked. Today, that usually means slow-cooked or prepared. This recipe for a Crock of Herring requires a whole day for them to slowly pickle.
Introduction to Scandinavian Cooking, The McCall Publishing Company, 1960