Having owned a 03. V10 15 pass Stock roof w QuadVan's 4x4 TTB, 265's and 3.73's - No engine mods or tuner, light interior buildout, roof rack/basket, side ladder, stock bumpers. The lift was maybe 1.5" over stock:
13 years and 50k+ miles of driving / very little towing - I was VERY happy w that setup. Fwy mpg normally around 13.5 at 65 +/- mph.
Removed the roof rack and bars for a run to Loveland 14.9mpg over the 1200 miles at 70mph. Had CCV install their hi top, drove back and got nearly the same 14.8mpg w 80mph stretches on 80 in the mix.
So the V10 can get decent fuel mileage. Always felt like I had hardly any foot on the go pedal, it was perfectly setup for our usage.
Of course, if towing often and 5k lbs, I would think it prudent to change to the 4:10 or 4.30. But I would miss the lower rpms when running unloaded/not towing.
Beastie 3: 2002 7.3 EB Cargo: Agile TTB, CCV Mid Top, Custom Walk Through, Lots of stuff added. BlingMyRig.com