Hi Chucky, and welcome. It's good to see you here first, instead of after you made all the decisions. You'll get a lot of advice and opinion here, and in the end, I think you'll end up with what you need. So, here is my advice/opinion.
Get the 5th wheel first. If you can, get it weighed rather than depend on the paper weights. If not, use the GCWR of the trailer to determine how much truck you'll need. This way you will know for sure you have the right tool for the job. If you do it this way, you will have enough truck to pull the trailer you get when it's loaded to its maximum capacity.
As far as the truck itself, I know the Fords (F-250/350/450) CrewCab will sit 6 fairly comfortably, as long as you get the 40/20/40 bench seat in the front. The console version will only seat 5. And as you can imagine, the "20" in that front bench seat will be for a small person. Although my 6 foot son has sat there a number of times. For short distances!
4X4 will probably depend on what type of camping you do. If you tend to get away from the pavement often, or even on occassion, you might want to consider it. In my 3 trucks pulling my two different trailers, I have not had 4X4, and have only had one instance where I could have used it. And it was a very rare type of event. Again, it depends on where you go.
Dually will probably depend on the 5th wheel you end up with. If you stick to a 30 - 32 footer, it is unlikely you will need it, but if you get much bigger, or just happen to get a relatively heavy 5th wheel, you might consider the dually. Or if you just want to feel better about the added stability the dually will provide.
Now, I'm kinda partial to the Fords, so I will loudly tout the virtues of the new 6.7L. It is so nice towing my 5th wheel. It does a tremendous job. The power and torque are unbelievable. And as I said, I think they provide the most room, with the exception of the MegaCab in the Ram. I believe any of the new diesels will be fine, but I sure do like the Ford. And I recommend that you test drive all of them before you make any decision.
I hope this helps you. And good luck!