IMO, from what the OP is describing a gasser is the way to go. Over the last 10 years MPGs have improved a lot so there would be some payback to buying newer. Enough to offset the higher cost of newer? Probably not but it's still there. A lot also depends on how much daily driving is done.
The Chevy 6.0 is a really nice reliable motor in a 2500. I like my Ram 6.4 for commuting MPG and it's great for towing but will likely be at a much higher price point than the 6.0 Chevy could be had for. I was never a big fan of the Ford 5.4 - I had one and thought it was a total dog both performance and MPG wise. A buddy with the Chevy 6.0 ran circles around me.
Of course, a diesel is nice but not necessary and expensive purchase price for what you are hauling. If you do go the diesel route the 7.3 Ford was a great motor. The Chevy/GMC Duramax is great but good luck finding one. The older Dodge/Ram are great as well but hard to find.
A 2500 of anything will be night and day different from your Titan. The right truck (as in not too much or too little) for your current trailer is a 2500 gasser. You could always go for more but it's not necessary.
