Here is a recipe that I have used the last couple of times that I have put a pork loin in a slow cooker.  I am not certain, but I think I may have gotten this one from AllRecipes.com.   

Sweet & Tangy Slow Cooker Pork

1 1/2 cup catsup

3 Tbsp chili sauce

3 Tbsp red wine vinegar

1 1/2 Tbsp lemon juice

1 1/2 tsp Worchestershire sauce

1/4 tsp hot sauce  (I just added w/o measuring.)

1 Tbsp Dijon mustard (I think I used spicy brown mustard.)

1 1/2 chili powder

1 1/2 tsp garlic powder

3/4 tsp celery seed

1/4 tsp black pepper

1/4 cup brown sugar

1 1/2 red bell pepper, cut into rings  (I chopped them)

1 1/2 onion, sliced into rings (I used 2 onions, and I chopped them)

In bowl, mix catsup, chili sauce wine vinegar, lemon juice, Worchestershire sauce, Dijon mustard, hot sauce, chili powder, garlic powder, celery seed, black pepper, and brown sugar.

Place pork in bottom of slow cooker. Layer with onion and sliced peppers.  Pour sauce over pork & vegetables.  Cover.  Cook on high for 1 hour, then cook on low until done.

This recipe was for 3 lbs country style pork ribs; however, I used for pork loin.  Also, first time I thought it was a bit dry, so next time I doubled the ingredients from what is listed above.  That makes plenty of liquid.  You may want to try recommended rates first time, and then adjust in the future if you see the need.