three thoughts for this assuming the oven is the only option
1) misto or pam spray oil. coat them - not just the pan
2) haven't tried this but I wonder if you could not try the double fry method in the oven - i.e. bake them misted with oil. Take them out, pat them down then re-coat and bake a second time.
3) This may be your best option but I have not tried it either, not with sweet potatoes at least. The way that I was taught to make Eggplant Parmesan involves slicing the eggplant, laying the slices out on paper towels or similar and then salting them and covering with paper towels. Apart from the flavor, this reduces the water content and then I fry them up and they are crispy rather than soggy messes. Since I believe the issue here with sweet potatoes is that they are denser with higher water content (thus harder to crisp) this method may work YMMV. And for the same reason, I think thinner is better whether it be a chip or a fry. More surface area to crisp, less density. So ideally mandolin style thins.
And for option 1 and 2 I think pre salting would be helpful for the same reason. And when laying them out, don't let them touch each other. The more surface area exposed, the more can be crisped.
Coating them is more like a crispy batter IMHO. Like zucchini stix. Still a lot of moisture inside even if the batter is crispy.