In a small bowl, whisk together tamari sauce, maple sugar, salt, garlic, onion, chili powder, ginger and pepper. Slice sweet potatoes in half lengthwise and into 1/8-inch thick strips. Toss in a large bowl with marinade. Let sit for 30 minutes until wilted and pliable.
Arrange slices in a single layer on 2 parchment lined baking sheets. Bake at 350F for 20 minutes. Flip slices, then reduce heat to 175F and return to oven to bake further for 1 hour to 1-1/2 hours until dehydrated and chewy. Remove strips to cool as they are ready. Texture should feel dry and leathery.
Let cool completely before storing in an airtight container for up to 1 week.