In the same van & given reasonable, sensible driving a 2.5diesel could well give better mpg than a 1.9diesel & same if you compared petrol engines of those sizes. This is due to more powerful engines doing less work for given weight of vehicle. This is comparing same vans of same age fitted with different engine options.
Even then it might not be case if comparison is between turbo & non turbo diesels so vehicle details are required for more specific advice.
------------- Thank you Donnod123
------------
Coleman Evolva (might have more than one shhhh)
And now a VW T25 high top.
Tilley Titan
Tilley Trio
Cobb BBQ
We have a 2.3 JTD Ducato van and are getting around 29 MPG. Depends on how you drive it but I try and keep the rev counter at 2000.
Our last van was a 1.9 Ducato non turbo and we were getting around 23 -35MPG
If its same size, same age van with choice of 1.9 or 2.5 then go for 2.5. Presumably this is 2002 or earlier? both old technology indirect injection engines could be turbo or non turbo. With older engines its even more of a case on 'big engine, light body' gives better mpg.
Old technology diesels give poorer mpg than modern common rail but much cheaper repairs on old type. If motorhome is 12yrs old or more a less stressed larger engine will be better bet for longevity than smaller engine.
Hmm
Well for me
If you are you do short journeys at lower speeds then the smaller engine
If you are going to want to do longer trips higher speeds then the lager engine. It's normally about keeping those rpm low..
And an auto ( torque converter type) will decrease your mpg. But if you can get one with a manual box which changes automatically this is better.
Good luck with the choice
------------- Thank you Donnod123
------------
Coleman Evolva (might have more than one shhhh)
And now a VW T25 high top.
Tilley Titan
Tilley Trio
Cobb BBQ
If it is any help my 2.8 Ducato averages around 30 mpg, but I do tend to stay below 2500 rpm. Like chelseafc in the earlier reply, I had a MH in the USA, and from the 7.5 litre petrol engine I would get 7 mpg on a good day.
If the 1.9 is without a turbo stay away. I had the engine in a Citroen Relay short wheel base van and it was dreadfully slow even without all the motorhome stuff bolted on.
That engine was pretty useless in a Peugeot 306 as well without a turbo.