The pin weight for the Open Range Light I am looking at is 1265 lbs. The gross weight is 9520 lbs. Nothing in the truck except me, my wife, and dogs. The bed would necessarily be dedicated to the 5th wheel hitch.
Pin weight is typically 20-25 percent of gross trailer weight. I doubt it would be 1265 pounds. Never use brochure or dry weights. They are for selling fifth wheels to people that don't want to see real camping ready numbers.
Again, if you love the megacab and I cannot see why anybody would not, get the 3500 dually and you will be just fine.
You can go online to the Dodge Ram towning guide and see for yourself how marginal the payload is for the 2010's 2500 megacab. That should give you a good idea of where you are as to being under, at, or over the trucks gross weight.
Somebody else suggested a bumper pull TT, that would work just fine.
