mpg is always tricky issue.
I've been recording mpg on my vehicles my whole life, even in the past androids and fuelly.com did not exist and I had to carry notebook.
In highway cruising crucial is frontal area and speed.
Even with my 12' camper I can get 30 mpg and I think 4 mpg would be manageable too. All depends on speed.
Sure we can talk abut different needs for different climates.
In Las Vegas summers, I have to wear long pants and gloves.
Touching parked vehicle door, or seat with bare skin will give you 2nd degree burn. No worry about diesel gelling , but gasoline would blow its light fractions in parked vehicle in just few days. Than I would have to set a tent with cooler in it :)