preventing pipes on an outside wall from freezing
You have to get insulation between those pipes and the outside wall of the cabin and lots of it.  Don't insulated the warm side of those pipes.  That is the only heat that keeps the pipes from freezing.  I wouldn't pursue the heat tape idea.  Put all of your effort into insulation the cold side of those pipes.  And keep the interior of the cabin above 60* all year. You will eventually get it.  Good Luck. You can always let the water trickle from a faucet on those cold and windy nights. Additional thought, in general the exterior wall of he cabin must be well insulated.  Hope this helps. Good Luck.