I have a 38ft class A gas, gets 6.5 to about 8 mpg pulling my wrangler 4dr. Speed, wind, hills play a big roll in mpg thus the big variation in mpg. My MPG is within .5 mpg going 55 or 65 mph & I think the Allison 6spd trans helps a lot there. When I was RV shopping last summer I drove a providence 39s which was in rough condition but went down the road much better then a gasser. My worry was maintenance costs & having to buy used DP vs new Gas. So better interest rates on new we went that route.... we figured we will also only put 5K miles or so a year on the RV which also made us look at gas.
I do always talk to DP owners when out camping & most seem to state they get around 8 mph on their 38-40ish foot RV with the 43-45ft ones getting 7-8mph.
I would mostly look at the floor plan you want (which was another reason we purchased the RV we did) & let that decide for you. A 31ft class C doesn't have close to the room of a 31ft class A. You loose 5-7 feet due to the hood & all that stuff which is all living space in the class A.
