Boy oh boy wait until you see the posts about this... LOL truck wars, here we come
For us it was a Dodge 3500 Dually Diesel - the 2016 builds have the largest cargo carrying capacity in class (and as a family of 5 you will need that!!) and it will pull your Sandpiper with NO problem (it can pull 29K lbs!) The dual rear wheels will give you the stability you need and want. The 2016 models have a higher cargo and tow capacities than Ford F450s
4x4? Maybe. You can carry a few hundred LBS more without the 4x4 but when you need a 4x4 you really REALLY need a 4x4 so for us that was a no brainer.
I would steer away from the megacab (all it gives you is reclining rear seats) because you loose a LOT of bed space - but YMMV what with 3 kids in the back seat!
Consider putting a "Fuelbox" (combo aux tank+toolbox) in the bed and you will be good to go to find your best price for diesel!
Oh, and the 2016 - which is just now being released is indeed significantly better than the 2015 builds!
Good Luck and PLEASE keep us all posted on your adventure - sounds wonderful!!