In our various 2.8 Jtd vans we got around 400 miles per tank full, this one driving steady as I always do as we're on holiday I get 450 to 500, so it still seems low, would lack of a service make that much difference? Never serviced a van ever, I won't buy one unless they have a fully stamped up book, this one is an aberration to that rule
We have the 90 litre fuel tank, Kev, and I work on a safe 400 miles before refilling; this equates to the fuel gauge sitting on the edge of 1/4 full, but the refill is usually no more than 63 litres. I've averaged 29.46mpg over 9723 miles, and, like you, drive around the 200 to 2200rpm most of the time. The engine had covered only 8767 miles when we bought Brunhilde at 5 years old [very little service history other than the Hab Checks Reports!], and the engine was really tight. It loosened quite a lot [relatively speaking] at 10,000 miles and again around the 15,000 mark. She's covered 18490 miles now [recent mileage has just been the 25-30 miles engine warm up runs], so I expect the current tank mpg to be very low, and the engine to free up a lot more once the 20,000 miles passes.
I find that the engine feels sluggish, even when the temp gauge reads warm after 10-15 miles, and then at around 20 miles, it suddenly wakes up and pulls much better [I keep the oil temperature gauge on display in the car, and after 20 miles the oil is around the 100 degrees mark].
I am paranoid about servicing schedules, both distance and replacement of filters etc. The almost 10,000 miles we've covered since purchase have been 6 journeys of around 1000 miles to France and Spain and then a number of relatively short trips to the next Aire, so the oil and filters will get a fair amount of crud building up. I expected to cover 10,000 miles per annum, but Covid/buggered ECU and last week's health problems have kept the annual mileage down to around 6,000, making the £375 main service more expensive per mile than I anticipated, but still a sound investment for peace of mind!
If I could mprove my stress per gallon to over 12, I'd probably be a wee bit healthier ...
Steve