If you've got a little extra time, you might skim through the "Diesel vs. Gas" thread. Lots of opinions but some real good information in there.
Personally...I went from a truck very similar to the one in your signature (I even had the Tekonsha P3 and still have the Equalixer hitch) to a 2500 HD Duramax. I also had the idea of going to a fifth wheel but in everything we liked the pin weight put us too close to the max payload for my comfort. We've settled on a 35' TT, 8,500 (ish) dry.
I know gas is cheaper than diesel, but over the past 4 months and roughly 3,200 miles, doing the same driving I always did (no towing yet)I'm only seeing about $60 per month additional cost for fuel.
For what it's worth, there's not a thing wrong with the 6.0l motor. I looked real hard at it but my circumstances made diesel a good choice for me. That's pretty much what you'll have to decide...what fits your circumstances?