Whisk flour, oats, Truvia Brown Sugar Blend, cinnamon, and salt in a large bowl. Dice butter into small chunks and add to the mixture, cutting it in with a pastry cutter until the mixture is crumbly. Stir in chocolate chips, if using. Set aside.
Rinse the frozen fruit to get rid of any ice chunks or crystals. Place in a second large bowl. (You can also use fresh fruit, but if doing so you may want to reduce the cornstarch amount to 1 tablespoon.)
Sprinkle the Truvia Baking Blend, lemon juice, and cornstarch over the berries and toss to coat.
Spray the bottom of a 5-7 quart slow cooker with nonstick cooking spray. (You can also use a slow cooker liner, but spray it with nonstick spray as well.) Add the fruit and juices, then top with the crumble topping.
Place a paper towel over the slow cooker opening, then cover with the lid. Cook on high power for approximately 2 1/2 to 3 hours, or until the top is wet looking (no powdery mixture remains) and the edges are bubbly.
Serve warm with low-sugar ice cream or whipped topping.