Can’t do spreadsheets so I knew there was a reason I didn’t worry about the fuel consumption.
Of course, Pepe, you are missing one obvious trick. Make sure all your journeys are downhill and coast. If the place you are going to is uphill, choose another place. Your relatives may (or may not) appreciate the lack of visits if their home is geographically challenged.

2002 LR Discovery 2, diesel manual & Eriba 530.

So far, with and without I’m getting a similar MPG, which is between 25 to 27. I generally don’t exceed 60mph whilst towing, and when I’m not towing I generally top out at 65 MPH unless I’m in a hurry.

Which is why Eribas are proving popular with electric towcar owners - both for their lighter weight and for smaller frontal area with better streamlining. We'll probably see a trend towards even more pop-up-roof caravans and more aerodynamic profiles. And the return of the smaller Eriba models that have slowly disappeared as owners sought more and more space and luxury. Lighter materials may well be used in construction, though they are pretty good now - it's just all the extras and paraphernalia we load them up with that doesn't help! Perhaps an aluminium chassis design? Or a carbon fibre one?

Our VW Golf 1.4 petrol amazed us by doing around 40mpg on a trip to Scarborough with our little Puck.

On our 3 week trip, 1466 miles majority towing, averaged 33mpg with our big tractor. Very happy with that

Recently back from a 2000 mile trip from North Wales to the Ardèche & back with our 1.5 Skoda Kamiq/Puck L combo. I don't spare the horses on the péage so "2 cylinder mode" was rarely engaged which gave a return of 32mpg. Not as frugal as the 2ltr Skoda Yeti we had last outing but there was no need to hug the "Lane Of Shame" on the long inclines with the artics this year .

Just returned home from Scotland - 215 miles of mixed A roads, bit of motorway and minor roads. 37mpg. That's with a 2006 Volvo V70 D5 with 155000 miles.
I used to tow a big van with a two litre petrol Saab 9-3. Once when towing from Yorkshire to Norfolk into the wind it gave 12mpg. Didn't keep it for long after that....

VW Golf 1.4 petrol towing our little Puck, average 40mpg

1.6 Skoda tdi Estate. 5 speed, 2013 with 120000 miles, at 60mph = 62 mpg , pulling 1100 kgs = 34mpg