I helped a neighbor replace the spring in a manually operated Carefree of Colorado awning. Different from a Dometic, but what we did to wind it was use half of the rafter as a lever. Since it was already attached to the roller bracket, all we had to do was wind it up, the specified number of turns. Then we slid the rafter back together. EASY!!!
You may be able to Un-Wind the same way. If not, a pair of 10" ViseGrips is enough. But wear gloves and goggles and be ready to bail out if it slips. If it gets away, you won't be wanting to try and stop it.
That Motor Assembly is VERY expensive, $300+ if you're lucky. So take it apart and see if you can replace less than the full kit. You may get lucky and find a sheared drive pin or the like.