I have a 1997 Hymermobil on a 2.5tdi Ducato, last year when I 1st bought it I got about 27-28mpg. Went to Germany & France and got about 30-31 on average.
This year I put Continental vanco campers on, fitted cruise control, put Synthetic oil in & drove to Italy & back (through the old roads in the Alps not Autostrasa etc) the van was fully laden including 1/2 tank of clean water (to start later in the waste water) & 2 11kg gas. The overall average was 32.8mpg. Surprisingly the stage through the Alps low gears etc returned just under 38mpg.
What dragged it down was the standing stop start on the M25, & a couple of long delays on the A1 & A22 in Italy otherwise I think I would have got nearer 34mpg overall.