Answer: A Dodge Cummins turbo diesel motorhome appears to get around 14-15 mpg. Here's a story:

http://fostertruck.com/cummins/102000update/default.htm

And here's a company that does new conversions of both Sprinter Vans and Dodge Cummins pickups:

http://www.vancamper.com/index.htm

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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