Not sure what the dealer means by a pivot type kingpin... but if he's saying you would need that in addition to the slider, then I call BS on that.
A slider should be all you need. I have a slider on my 6.5' bed, and have used it maybe 2-3 times in the last 3 years... and that's not because I actually needed it, but as a just in case type of thing while backing up sharply in uneven terrain. I can't think of a time I've ever come anywhere close to hitting while moving forward even in tight spaces, so his gas station/moving forward comment I would also call BS on.
The actual length of your bed and the corners of your 5'er will help determine if you need a slider, but it's good insurance to have. The measuring recommended by Biscuit sounds like a clear way to determine if you need one.