Reduce the heat to medium, add the onion to the skillet and cook until soft, about 5 minutes. Stir in the garlic, can of hot Rotel, cumin, and chili powder and cook 3 minutes. Increase the heat to medium-high, add 1 1/2 cups water and can of original Rotel and simmer, scraping up the browned bits from the bottom, about 3 minutes. Transfer to the slow cooker, cover and cook on low for 7-8 hours.