With "RARE" exception to the RV itself like a 1992-1995 Telstar motorhome, I would avoid an old Ford chassis that did not have the V10 engine and at least a 4-speed transmission.
For reference regarding fuel economy, our 2007 E350 with the V10 engine and the TorqShift 5-speed transmission of that era
(MANY PICTURES HERE) averaged 10.5 mpg, a figure accurately logged and calculated upon returning from a ~5000 mile RV trip (not towing). 10.5 mpg is a whole lot better than a mere 6 mpg.....and there are other matters pertaining to performance, behavioral, available parts, and reliability.