Diesel will give you around 50% better mileage (12 mpg versus the 8 mpg you're getting now). I had a 2006 5.7L Hemi with a similar TT to your current one, I would get anywhere from 8 to 9.5 mpg. When I went to a diesel I would get 12-13 mpg with the same trailer. I get that same mpg with a heavier fifth wheel, wind is the biggest impact to my mileage not trailer weight.
Really i'd say it depends on if you are able to do a straight trade across from your current truck to the diesel. If you have to pay any money for the diesel then it probably isn't worth it. Put that money into gas instead.
Diesel torque/towing will be a huge improvement over a 2006 5.4L for anything relatively heavy... your trailer isn't relatively heavy. 7.5k GVWR is nothing behind a diesel truck, if you like to drive and not notice you're pulling anything then diesel will give you that with your current trailer weight.
I would stay away from Ford diesel trucks, the years you'd be looking at had a lot of issues. Others will likely chime in on the 6.0L Ford versus the 6.4L ford (and maybe even the older 7.3L). Chev/GM and Dodge in those years should be fine.
8 years old should pre-date all of the extra emissions junk, 10 years old definitely will.
I'd personally make sure the truck has never been chipped, others may disagree with this statement but I know too many people who bought used diesel trucks that had been chipped and had a ton of issues with them.