if it's for living- then mpg doesn;t much matter.
however, the motor will be early EFI and less efficient than a 99 or 2000 +. and a 3 spd or maybe 3 spd with OD tranny, the rear axle ratio will also have a huge effect on the mpg.
I would guess at 65 mph you might see 10~12
for living, take the time to check the rv systems and roof. interior for signs of leaks
I think I saw the ad for this rv- I'd take the mounting of the stereo in the dash as an indicator of how it's been taken care of and the standard of work the owner is willing to accept- nice artwork too!