Best MPG

The one that is parked ;)

I get 24mpg.

Mercedes Sprinter based Class B or C or those advertised as B+. Perhaps also the Winnebago Via which is a Class A.

If looking for an older unit, then perhaps a used Winnebago Rialta based on the VW VR6 engine.

For one or two people, a motorcycle with a tent.
More than two people, a Prius with a tent.
Seriously, depends on what you planning on doing. Long trips, lot of cargo, how many people, and for how long.
Sure, a small van conversion is better mpg wise, but if your carrying 3, 4, 5 people on a long trip, you'll obviously need something a lot bigger.
You need to define your needs before looking at something just for "the best mpg".
Grumpy

I have a class A 40' DP pulling a toad. 55 - 62 mph I get 9.2 avg. Worst tank ever, high head wind 6.2. The pedal makes the difference for me.

if mpg is your goal? find a sprinter with the I 5 mercedes benz diesel engine. my log book shows nothing under 20mpg. my MH is a LTV freedom serenity II two people unit with plenty of space and a full bed. had class A,s and won,t go back.

The one doing 55 on the highway.

RV's don't get "good mileage."
But, the lighter, smaller, lower they are, I guess they will get "better" mileage than big, tall, heavy ones.

Maybe look at something like this......Roadtrek...LINK

Class B is probably the best. But even then dont expect much better than 10MPG