I have the Advance Truck Bed Cover. They look and work like the competition's Fold-a-Cover. They are available on the web, and the website has a dealer locator link:
http://www.advancecover.com/
I love mine. I originally bought it in 2003 for our 1999 PDS. I installed it myself, and then transferred it over to our 2005 PSD.
I've had no problems with it. It allows me to lock and secure stuff in the truck bed when we are not pulling the 5W. And when I'm still hooked up, but at a campground, I pull it back so two panels cover a goodly portion of the bed. It keeps most stuff dry when closed (water-resistant). I'm sure if I did a little adjusting it could be actually waterproof.
With these covers, they do make it a little more difficult to see the hitch when backing up to hookup. But I just taped a white tie-wrap forward of the hitch pin, and that solved the visibility problem.
Overall, I wouldn't do without one of these covers!!!!! Excellent.