I had a box on my old 120 57 plate, all it did was boost fuel rail pressure, and trick the computer in thinking it was 3-4 avg mpg better. Once I removed it, mpg figures went down 3-4 mpg, yet fuel usage was the same as with it fitted!
Based on doing same 95% motorway delivery run 5 nights a week.
I would recommend a remap any day, why... Because a proper remap company, super hips did my 3.0 180 in 2012 at their Buckingham head office, spent a day on it as it was the first 180 they'd seen, no base figures so they went to town on her,
A good Remap caters for your van, not an average 3.0 160 (in your case)
Taking all parameters like, temp's, speed, air flow, throttle position, revs, etc etc and they can set power/torque correct for your and vans needs, as opposed a box that only boosts fuel rail pressure,