I like to use Golden Mushroom soup with mine, unless I'm making them Mexican Style, and then I use the Nacho Cheese Soup. I have also used tomato soup, and even salsa.
I haven't tried doing them in the crock-pot, but I don't see any reason why you couldn't. They're just not going to need to cook all day. I would suggest putting some sliced potatoes or a boxed rice mix in with them (around the sides and on the bottom), so you'll have your whole meal ready. If you use the rice mix, you'll need to add a little less than the water called for in the directions, as you'll get some water out of the peppers.
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