5.4 E350 performance and reliability?

What kind of reliability and fuel mileage does this produce?

reliability...great.
fuel mileage...not known for good mileage. Same as V-10 IMO.

Ours leaves driveway at 12,000 pounds. We use ours all over Pac NW, which has a lot of mountain grades. But for sure not a ball of fire but has never failed us. Depending on the year model (same as any motor) has its problems, but once understood no problems. Oil of 5w20 surprised me at first, thought the 5w20 was a typo at first. The oil change place I use... used a no name brand oil filter. I now bring my own filter. I use motorcraft. Make sure you use an oil filter with "silicone valve" built in. Prevents oil from leaking down. Which means when starting engine you are not doing a "cold start up" when you start it.

Lots of info on the Triton motor available, good dependable motors.
https://en.wikipedia.org/wiki/Ford_Modular_engine

About 110,000 miles now. Fouled injector at 85,000 miles, not sure if bad gas or what? I now carry spare fuel filter and the little disconnect tool set ($15.00) to get filter off and takes about 5 min to change and a spare injector ($70.00) about 30 min to swop out. Also carry spare cop (coil over plug) but never needed one yet. Ford says some fluids are good till 100,000 miles, but I would do fluids sooner. Of course at 12,000 pounds keep up on brake pads. Front brakes real easy, once you do them once. Rear brakes, a pain being there are duels. I have the older 4r100 trans, newer ones better IMO