5.4's, or any Triton, love to rev. Fuel mileage doesn't change with them whether you rev or lug. This has always been the case. The V10 is just like the V8's. They are designed to rev and perform in all circumstances at higher RPMs where they make the most torque, fuel mileage irrelevant.
The difference in V10 vs 5.4 is that you can go faster but you'll get the same mileage, generally. The V10 drinks more but works less, 5.4 works more moving a brick through the air.
I've never gotten more than about 13 on a built out camper van with a 5.4 and I've had several. I've also had several V10s and 10-11 is the norm with 12 being on the high end.
The best fuel mileage I ever got was with a V10 was with a 2wd ClassB+ (read lower and more aerodynamic). It was a 24', 8' wide and 11' tall and weighed in at about 13k. It still managed about 12 mpg driving 70+. The advantage was it's aerodynamics.
Here's another interesting point, one that is true though I still have trouble believing what the calculator told me. I once pulled a Jeep Wrangler on a 5k mile trip around the country with a 2wd 5.4l camper van. Fuel mileage was 1 mpg less for the entire trip! I don't know how but it's true. I can only surmise that as stated above it's more about aerodynamics than anything else. The Jeep sat in the shadow of the van where a box with lots of frontal area would've changed things.
I would love to do as much field testing with the 7.3 gas as possible to compare to all the V8's/V10's I've owned. If one of you could drop one off it would be helpful. PM for address.
How can Ford be putting out something that gets 7-8??
Because we're not talking about SUV's, we're talking about big boxes that you can walk around in. Completely different animal.