Adding to j-d here with personal experience.
Our 2007 E350 with 6.8L-V10 engine weighs 9200 pounds (10,200 loaded up) with the house on it's back. If we didn't tow a vehicle, I feel the smaller 5.4L-V8 engine would offer plenty of power to benefit from a 10-15% gain in fuel efficiency, and more than a $1000 savings in the purchase price. When we don't tow, we have more power than we really need. In-fact, I wonder if the 5.4L-V8 would be adequate even when towing. How fast do you want to climb up that steep grade anyway? We would never dart around in traffic with either engine. If in a hurry, a motor home is the wrong vehicle of choice.
I do think our aerodynamic shape, low height, and narrow width all help a bit with my experience.