Hi Gumbo
I'vew had a couple of MHs and you will probably be looking at low 20s if not high teens depending on size of the van , the engine size and bhp.
I had a 5 1/2 m Ace Capri with a 2ltTD (100bhp) Fiat engine , in MH terms it was a fairly light weight coachbuilt. That got about 22-24 (few years ago now!)
We changed that to a 7 1/2m Hymer Coachbuilt which was a fairly heavy medium sized van. That was a 2.2lt TD (140bhp) Ford engine and we got 20-22 from that.
My boss has 3lt TD (Automatic) Renault based mid sized low profile jobbie and she recons 18-20mpg.
If you go down the MH route you wouldn't get good mpg unless you look at the panel van conversions and then you still have to take into account the added weight of the conversion.
Low 20s should be you average guide.
Its a very difficult subject as conversion , size of van , size of enige and BHP will all be factors in mpg (much the same as different car and caravan combinations.)
As an aside I still miss my MH days , we used it probably 2-3 times more than we use our caravan. Despite purchase cost and fuel costs and some of the mobility issues you get I still think we got better (useage) value for the