Welcome!
So as you’re aware, your truck is well suited for that size trailer.
Whatever you get with your gas truck will be similar with others. Maybe a shade better with a big gas engine and deeper gears but not much. 1 mpg maybe.
Diesel will do about the diffenrece in the energy density between the 2 types of fuel. Or about 30% better. Again, maybe more because a big diesel wouldn’t be breaking a sweat with that trailer. Real world driving the speed limits I’d expect maybe 12avg out of a big diesel.
Haven’t pulled with or even driven a 1/2 ton diesel pickup though.