Maurice,
I have towed 4 Eriba's since 2003. 1000kg Triton, 1300kg Troll, 900kg Familia and currently 1200kg Triton. [all 3 berth vans]
My mpg has differed little when towing with a 100bhp diesel VW Touran and 110bhp diesel Skoda Yeti over long dual carriageway/motorway runs, and it is only on shorter runs the heavier vans have an impact.
I have recently swapped to a 1.5 petrol Audi A3 with 150bhp and 250N/m torque. I was expecting a big difference, but I have been proved wrong. I am on a par with real brim to brim mpg of previous diesels although dash computer shows I'm down a little.
On recent near 2,500 miles towing around France I returned 33mpg with fully laden car and van. On a lightly loaded run this summer I recorded 37mpg out and 35mpg return. Some owners achieve 40mpg, a figure I think is realistic with a good modern diesel and something I saw when driving like a vicar with the Yeti.
Colin