Unless you can trade straight across then spending more money to save on fuel won't work. You'll definitely get better mpg with the diesel. But remember gas prices typically drop in the winter and diesel stays the same. Lately it seems that during the summer they even out.
For me my break even point is when diesel is .50 or less and when towing and non towing @50/50. That's coming from a 2010 F150 5.4 3.73, towing a 31'TT @7300lbs getting 8.5 and switching to diesel and getting 10.5. We now tow a mid profile 28' 5'er @9000lbs and get 10.5-11.9.
I save enough on one single 200 mile RT in fuel to cover the extra cost for oil changes.
8-10 year old diesels IMO would put you in the Dodge 5.9 or Dmax. 04-06 Ford 6.0's seem to have more issues than the 5.9 or 6.6. I know one thing for sure towing with a diesel in a 3/4-1 ton will be a lot nicer than the 5.4
Couple nice buys in my area.
http://salem.craigslist.org/cto/4367565243.htmlhttp://salem.craigslist.org/cto/4418257425.html