Spray inside of a 4-6 qt crockpot with non-stick cooking spray or line with parchment paper.
Remove biscuits from can and cut each into 4-6 pieces depending on size. Toss pieces in a bowl with 2 tbsp olive oil (or melted butter) and Italian seasoning. Add more olive oil if needed.
Place a single layer of biscuit pieces in the bottom of your crockpot. Top with some of the shredded cheese and pepperoni. Continue to layer until you have used all of your ingredients. The final lay should be cheese and pepperoni.
Cover and cook on High heat setting 1 hour 30 minutes to 2 hour. (Not all slow cookers cook the same. Test for doneness after 1 hour and 30 minutes by inserting knife into center to make sure biscuits are fully baked; outer edge will be dark golden brown.)
Allow to cool for a few minutes. Pull apart and serve with the marinara sauce.