Why not get a 2013 gas powered 1-ton long bed for a $1000 more and have the more economical truck to drive (diesel is more expensive than premium gas in most parts of the country). As advised start with the camper first. Unless you want to get a 11' camper for a dry bath the Duramax is a nice to have and not a need to have and the same applies to the DRW.
GM made a great many changes to their heavy duty trucks for the new platform (2011-2013) and the 3/4 ton can take a heavier load more capably than the older trucks SRW or dooley. The gas powered truck will gain 800 or more pounds of load capacity with the lighter engine. 1-ton only adds an extra set of leaf springs and the rear brake rotors are 4% larger in diameter - not a lot of difference which is why the dealer invoice is difference is going to be less than $700.
Time to take a deep breath and step back as neither truck is your best available option.