Is it a diesel? I also used to get more to the gallon. Two possible reasons I think:
Possible higher spec fuel. Certainly French diesel smells different, and I've noticed that truck (ie normal) diesel is often a bit cheaper.
Long motorway journeys with much less accelerating and braking than on UK roads.

 

What car, let alone DS returns 60mpg whilst towing? Is this a PHEV?

 

2020 2.0 Diesel HSE..
60 MPG was solo not towing (i wish)