Get the 3500. It shouldn't be much more money. Add a thousand or so pounds to that 14,000 lb. 5th wheel you're looking at. The dry weight on my toy hauler is 12,400 lbs. Here's the weight ticket on my truck and trailer from a couple days ago. And, I didn't have toys in it, just a full tank of diesel in the truck, 1/2 tank of gas in the trailer, bikes, grill, 4 people in the truck, etc. It adds up quick.
I'm overweight by 200 lbs. on my rear axle. Heavier rated tires may be in my future, I don't want to but a new truck. The new trucks have a higher payload than my '07 but I'm glad I got the 3500 when I bought my truck new.