I have one of these on each side of my truck bed:
http://www.buyautotruckaccessories.c...sored%2BSearch
It swivels under the truck bed when not in use but I don't think it would work as well with a dually because on either side I use the top of the tire as the second part to stand on when reaching into the sides of my truck bed. With a dually the outside tire and fender sticks out too far and it may not work as well.
I have seen the running board extensions that goes from the crew cab to the front of the rear fender. I think that would work best and look best but might require you to completely replace any existing running boards if you can't find matching extensions for what's currently there.