I agree about the relative weight ratios making less sense when towing with a van. Also the comment I made about the aerodynamics being different is relevant. Another consideration is that a van is generally geared differently & the commecial diesel has tons of low-down torque.
My wife's Vivaro has the 115PS engine. It will tow our Charisma quite happily in 6th gear from about 52mph on the flat, & will quite happily take on all but the steepest of motorway gradients. It will pull from 40mph in 5th, so on open roads & motoways it is an easy tow. We have friends who have the long wheel base 2.5 litre 145PS engine, & that tows like an absolute dream - it does drink the fuel somewhat though.....
Jim