The Fiat Ducato produced in Europe has three roof heights; low, medium, and high. Ram sells the low and medium height versions. Ideally, the high roof version that offers a maximum internal height of 7' 1.5 " (85.5") would be better suited for a van conversion. Installation of a subfloor in the rear of the van, 8 inches high, provides additional space for tanks or storage while still maintain the 6 foot plus head room.
http://www.autoexpress.co.uk/fiat/ducato
This second link provides Fiat Ducato specification including Fiat's mpg ratings (note these are imperial gallons, divide by 1.2 to convert to US mpg).
http://www.fiatprofessional.bg/models/d ... ch_Spe.pdf