A 31' coach with the V10
could get 9 MPG, but you'd have to drive pretty conservatively. 7.5-8 MPG is a little more realistic for most drivers.
48,000 miles is nothing for the V10. It may have some age issues (fuel pump, brittle vacuum hoses, rusty brake calipers etc) but really that's only halfway to the first tune-up. I have 410,000 miles on my V10, and it's been run VERY hard most of the life.
One thing I did not like about the FunMover 31' is the short wheelbase. I think it's only 208". That means any cargo weight in the garage or trailer tongue weight will move lots of weight off the front axle, and it would be easy to overload the rear axle. The newer 31' class C's all have a wheelbase 220" or longer.
2000 Ford E450 V10 VAN! 450,000+ miles
2014 ORV really big trailer
2015 Ford Focus ST