Spray the slow cooker with non-stick cooking spray and add the cubed Velveeta and cream cheese.
Add the Monterey Jack cheese, pepper jack cheese, milk, garlic powder, and cumin to the slow cooker and stir to combine.
Squeeze the thawed spinach in a clean kitchen towel to remove all excess moisture, then add it to the slow cooker along with the drained diced tomatoes with green chiles and diced green chiles.
Cover and cook on LOW for 1.5-2 hours, stirring every 30 minutes, until all the cheese is melted and the dip is smooth and creamy.
Stir well before serving and switch to the warm setting to keep the dip at the perfect consistency throughout your party.
Serve with tortilla chips, pita chips, or fresh vegetables for dipping.