Engine isn't the main problem
The Ford V10 is a fairly efficient engine, and can deliver decent MPGs as long as power requirements aren't too high.
I just drove from south Florida to Texas in an E-350 van with the V10, typically around 75 MPH. It took three tanks of gasoline, with the last one going 415 miles on 27.456 gallons, for an average of 15.1 MPG.
Obviously my van is much smaller and lighter than Class A motorhomes, but it shows that the V10 is not inherently a gas guzzler like many report.
The biggest problem is in how much power the V10 is asked to make, not on RPMs or poor engine efficiency.
Roughly for comparison:
Large Class A = 100 HP at 60 MPH
BSFC = .41 pounds/HP-hr
Fuel rate = 41 pounds/hr = 6.8 gallons/hr
MPG = (60 miles/hour) / (6.8 gallons/hour) = 8.8 miles/gallon
Smaller E-350 Van = 50 HP at 70 MPH
BSFC = .55 pounds/HP-hr
Fuel rate = 27.5 pounds/hr = 4.6 gallons/hr
MPG = (70 miles/hour) / (4.6 gallons/hour) = 15 miles/gallon
Notice that in reality a V10 is too large to operate as efficiently in a small and light-weight van yet it gets almost twice the MPGs. If my engine wasn't so large that it operated inefficiently (estimated .55 vs .41 pounds/HP-hr), and was downsized in proportion to required power (roughly half the engine size -- like in newer Ford Transit vans), I would get just over 20 MPG at 70 MPH.
There isn't that much wrong with V10 other than it may be too large to be as efficient in smaller motorhomes like Axis and Vegas. And if motorhomes are made slightly smaller and lighter (like Winnebago Reyo in size), then they will need smaller engines for maximum efficiency. Maybe that's why Ford will soon offer the 6.2-liter V8 in E-series.
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