The Ford V10 is no longer manufactured, so a non-issue when considering new motorhomes.
New E-350 or E-450 will only have 7.3L V8, which I would personally prefer over my V10, but it too won’t deliver 15 to 18 MPG in a Class C.
P.S. — V10 wasn’t that bad on fuel. In an aero and light van I have gotten as high as 17 MPG for entire tank when driving at around 55 to 60 MPH steady. I rarely drive that slow for over 400 miles though. Around 15 MPG is more common for me. We shouldn’t blame engine when real culprit is 100 square feet of frontal area.