You will want to blanch them in oil, water will just put more moisture back into them. so in coconut oil is probably your best bet. Although I think you could probably dry them out halfway in the oven and then fry them. I haven't tried this myself though, just an idea.
my perfect sweet potato fries.
Purple skinned Japanese sweet potatoes ( they are dry enough )
cold cast iron skillet
slice into pinky sized lengths
place in pan
cover with a cup of coconut oil (at least)
turn on medium high heat ( you do cook with gas right ???)
turn as the sides crisp ( start looking at 5 minutes)
remove from heat...
all other sweets are too wet... if you want to use them cook on much much much lower heat, say medium low for much longer.