A couple thoughts. The Ford V10 is a revving motor. In the rev department its more like a Honda than it is a typical burbling Ford V8. Let the engine spin up - it was designed to run that way. Also, w/ the gearing of the rear axle its made more for getting going from a stop than it is pulling you into your seat when you're already moving. The Ford E450 is rated to 22000 pounds GCVWR - it will handle what you're pulling.
You might want to run it w/ the Tow/Haul mode on and see how it works for you. Many would already consider the chassis 'hauling' once it has a house dropped on it. Add towing - you're just adding more of everything.