I hope I am wrong but from Howard's avatar he has a Benimar Mileo 294. I took this from the datasheet from Marquis Leisure...
The Mileo 243, 264, 286 & 294 models have a 60 litre fuel tank capacity. The Mileo 313, 323 & 346 models have a 90 litre fuel tank capacity.
Did say I may be wrong , figures came from Ducato manual
Patrick