for the drivetrain is all in front, especially with a cab-over design with in-cab access (so you can access both cylinder banks for plug changes and stuff), not having a super-long driveline would reduce vibration and wear, and it should allow you to lower the vehicle (improving center of gravity) and better weight distribution (for things like holding tanks and interior design).

Road holding and cornering aren't exactly high priority on a motorhome anyway either.

I honestly don't know why most larger motorhomes aren't FWD today unless it stems from a weaker transaxle and/or maybe it is harder to access for maintenance?