There is a lot of confusion around this question.
There are class C motorhomes (van front end)
Then there are super C's (kodiak front end or M1 light truck)
My coach is a class 8 Truck Conversion...
There is a big difference between a super C and a class 8 truck conversion. Mine is built on a freightliner columbia chassis, some are built on Cascadia chassis or Peterbuilts or Volvo's.
The MPG question is YES they do get better mileage.
I had a new Vectra 40' with a 350 CAT and towing my jeep I got around 6.9-7.4 mpg....now with my Class 8 truck conversion with 14K miles (not broke in yet) I am getting about 9-9.5 mpg and others I know in our truck conversion group who have rigs with 30-40K on them are getting between 10-10.5 mpg. They are much more aerodynamic than the busses . Mine has an MBE 450 in it so more power and better MPG.
No hump between the cab and the coach like some and the ceilings above the drivers seat are full height, in my case 7' tall. Both chairs move to face the back as extra seating just like a class A. The one big difference is having the engine in the front with regard to noise. Although mine is very well insulated, you still hear the humm of the diesel....which I love but it doesn't interfere with conversation. Servicing them is cheaper and no one trapes thru the motorhome to work on the engine. I get service anywhere the big trucks go so literally hundreds of places along the freeway.
Last year I pulled off on the way to florida and had an oil service done just like you do your car.....pulled up, pulled in and 40 minutes later pulled out. I've had 3 class A coaches and I do like them but Dynamax really hit a home run with power AND luxury. We have quad slides, stone heated floors, huge shower and even with full tanks (140 gal fuel + 100 gallon water) I don't even come close to the weight rating and carry capacity of the coach. We live in ours 3-4 months a year so we pack it up and after weighing it last year fully loaded I had 7,000 lbs to spare and that was with the jeep.