You may be able to get another couple of years out of the electrical cord by giving it a quick swipe with a cloth soaked in acetone. I KNOW, I KNOW it dissolves the vinyl but it does remove the dirt quickly. Most importantly, wipe the cord well with ArmorAll or Meguiar's rubber and vinyl cleaner and protectant after cleaning with acetone.
The best you can hope for is a few more years using these protectants after cleaning. I got 14 years out of my 30A cord and will be disposing of it this spring as I've had it with all the marks it's now leaving on the deck. Actually, 14 years of bird crap and UV is not too bad at all.