A boneless pork loin is the same cut of meat as a boneless ribeye in beef. Would you cook ribeye in a crock pot?
Pork loins are typically a very lean piece of meat. It is best suited to fast, high heat cooking methods such as roasting or grilling. Brining helps a lot with this cut.
If you want to cook a piece of pork low and slow in a crock pot, I'd go with a Boston butt or shoulder. This is the cut usually used for pulled pork. It has more fat and connective tissue so will provide a moist and tasty piece of meat following a long cooking period at low temperatures. It will get, "...all tender and pull apart-y.
"If you want to make an apple pie from scratch, you must first create the universe." -Carl Sagan