Here's a history:
http://www.vixenrv.org/vixen/vixen_chronology.html

It repeats the 30mpg claim.

I was thinking of Agency RV but it looks like it doesn't exist anymore.

I too was skeptical of the 30mpg. The owner was insistent it was correct. Lincoln Town Cars of the same era had the option of the same BMW diesel. I'd be curious as to the mpg in the Lincoln

edit--looks like the Lincoln/BMW diesel was rated at 27mpg highway from the factor with an automatic transmission.

__________________
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13