Rental Trucks on Ford Cutaway chassis seem to be 5.4L. I rented one awhile ago. It was in shabby shape. About the size of a Class C, not loaded as heavily, and I drove it around 250 miles. Ran well, climbed the interstate grades I found (but I was only going from Jax FL to Tampa). Definitely got better mileage than our C (profile) probably by 2-3 MPG.
5.4 sounds fine to me for the coach in question. Particularly with TorqShift transmission. Ours has V10 and does way too much shifting and I think it's the poor management of its 4R100 by the PCM.
But I'm with Ron - Put'er on a Chevy if you can. Only reason I could see not to would be if you can't afford the difference in overall length. Chevy cab is longer than Ford. Which means you also get to stretch out!
The game keeps changing with each side upping the ante at different irregular intervals. Chevy now offers a six-speed automatic. I don't think it's an Allison, but it has all the Allison/TorqShift "tricks." If it were an earlier year where Ford 5.4 had TorqShift but Chevy was only a four-speed overdrive, my recommendation would be different. It would be FORD.