You'll pay that much more for a diesel up front. If annual mpg went from 10 to 16 the difference would be about $1300 at $5/gl. The diesel purchase premium, used, is still worth more than [3] years of the "savings", likely.
Look for reliability first. MPG won't mean a thing at 5-7k annually otherwise. Look at 3-4 yr old half tons and don't worry about it past the above and personal preference. An aftermarket trans cooler (downstream of the in-radiator factory piece) with auxiliary filter, would be on my list of things to add.
Best tires [Michelin], best brakes [Brembo], best shocks [Koni], etc, would make a real difference -- if you are intent on spending money -- for, when the truck and trailer are well matched via hitch rigging (WDH; all it takes is a 350lb tongue weight to need one; look at Equal-I-Zer), the reduced effort and wear make premium parts worthwhile.
Weigh the current combination on a three pad certified scale. You might be surprised at how attending to hitch rigging properly makes a combination vehicle that much better to drive/brake/maneuver.
After that, some aero mods for the truck and trailer separately as would seem to work well for your use.
I'd also look at a one-ton or higher rated with a 12' bed and underbed toolboxes if that would eliminate the trailer. In that case, a pre-2007 Cummins with the man trans would be best. (See
CM PL Model beds).