I agree that a motorhome is putting more stress on the running gear than a pickup. But my unit is on an E450 not a E150 or E250 so some of the running gear is much more rugged. My experience so far is that the running gear is the most reliable part of the motorhome.
I believe that with proper maintenance that the V10 and running gear will go 200,000+ miles before any serious issues. You will have to replace things like alternators, water pumps, and perhaps manifolds, brakes, wheel bearings, but no major issues. Very few Motorhomes ever make to that point.
200,000miles/9mpg = 22,222 gals * #3.50/gal = about $77,778.00 hope I live long enough and have enough money to make it that far.
Go, see, enjoy the sights and have great adventures ;-).