I am running 23M/G ....on avarage in my 2001 4,2TDI with 332.000 km.
Hauling our 2500 kg horsetrailer (with horses!) ....I get 20M/G.
Hauling my 3500 kg trailer with my (a bit too heavy) skid steer loader (
4200 kg)I get 18M/G
Speed when hauling in Denmark is max 50 M/h.
When going 30-50 M/h (without trailer) like scenic roads or gravel, I get 25M/g.....so slowing down is the fast way to save fuel..
I am not sure about the effect of the tires. I run 295/75-16 BFG-AT
Regarding diesel vs petrol....I think they are both great engines...you just make a choice and love all the upsides

...and close your eyes for the downsides
When I drive the 4,7 V8 I love the quick response at low speeds, the smoothness, the sound.....when I pass 50 M/h I say there is no difference in sound or handling/driving characteristics
When I drive my own 4,2R6 I just love the sound when she fires up...ohh the sound of all 24 restless valves ..ohh yes I can hear them all singing to me

...after about 500.000 km totally driving in the 3 LC 4200 TD´s I have had....I "need" that sound of a good old 6-in line Toyota diesel...
When I tried the new 4,5 V8 twin turbo....286HP / 600nm....thats quite something completely different...try it!!!....-and more fuel effecient the the 100!
Well...I think they are all wonderful cars, I just hope you are happy with exactly the one you have....