Spray slow cooker with non-stick cooking spray; add Velveeta and cream cheese.
Stir in Monterey Jack, pepper jack, milk, garlic powder, and cumin.
Squeeze thawed spinach in clean kitchen towel to remove all excess moisture; add with drained tomatoes with chiles and green chiles.
Cover; cook on LOW 1.5-2 hours, stirring every 30 minutes, until cheeses melt and dip is smooth and creamy.
Stir well before serving; switch to warm to maintain consistency during party.
Serve with tortilla chips, pita chips, or fresh vegetables for dipping.