FWIW, I've been cruising at around 55-57 MPH for years in our 24 foot E450 ... never over 60 MPH.
The V10 just loafs along at these speeds in OD, even with the 4.56:1 differential, and we can't hear it under the doghouse. It holds it's own in the High Sierras, too, right along with all the diesel rigs - both commercial big rigs and towable RVs. I think the E450 does so well pulling at high altitudes because of it's agressive differential ratio combined with the V10's high RPM limit, whichs allows you to rev it up as needed to add horsepower. We idle it a fair amount and it idles with only a muffled swishing sound outside and with absolutely no floor vibration on the inside.
We also are very pleased with the V10. Ford got it right with this truck engine.