Mainly long trips = Diesel. For shorter trips and a daily driver the gas makes more sense. The diesel will get better mpg but it takes a lot of miles to make up for the initial cost and the maintenance costs. I have both a new 6.2 Ford gas and a old school 7.3 powerstroke and the powerstroke does about 1/3 better on fuel than the 6.2. I only do about 12k a year so another diesel wouldn't make sense for me.
Now for my question. Why a dually? Why not a F350 SRW with the heavy payload package. I think mine is rated around #10,800 which is plenty for most campers. Dually is good if all you do is haul but they are very un-handy when it comes to everyday use.