What happens is that when the cable is initially manufactured, it's wound onto a big reel when it's all warm from the manufacturing process. Then when it's re-manufactured to have the correct end, it's wound onto another reel, or sometimes bagged (like in a gunny sack).
What this does is get the conductors to lying a certain way in the cable. They stay that way for a long time and take a 'set'.
Yours has taken a good and contrary 'set' and that's what you are fighting.
The only way to deal with the 'set' of the conductors is to pull the cable out fully, then work it back into the storage area in as long and as straight of lengths as you can. Other posters above have already mentioned this.
It will take a few times of uncoiling it and re-coiling it to get it to straighten out. But it will. The Armor-All is a great idea; wish I'd thought of that with my trailer when it was new.
Just be patient. Also watch it as much as you can for these first few times of removing it so that you don't get yourself a cross-knot where one loop of cable tries to go over the top of another instead of just sliding by. In those cases, you'll have to shove the cable back in and work it past the knot to pull it out. Yep, it takes patience. But it will get better. Maybe not this year, but in one or two.
Also, as you lay out the power cable, take care to take out any twists in it; this will speed up the process of getting rid of the 'set'. Lay out any excess in a figure-8 rather than coiling it in a circle. Circular coiling is what caused your problem in the first place...
I've dealt with this kind of thing for many, many, many years in television and audio work. New cables (no matter how expensive) will NEVER coil up the way you want them to do. You always have to throw them out and coil them up a half dozen to a dozen times before they even begin to behave.
