The trip we have just been on we towed down an 8' x 5' x 6' box trailer with a GVW of 1580kg.
Going down it was full so must have been near its GVW. Coming back it was empty so probably weighed about 600kg
I was surprised at the lack of difference in consumption:
Going down full - 26.61 mpg
Coming back empty - 28.17 mpg
Admittedly, probably towing about 5 mph faster coming back but I expected the difference to be much greater.
The other odd thing is that the car usually shows less mpg on the display when driving without a trailer. However, with the trailer it showed more mpg.
"Don't think of them as problems, think of them as opportunities."
"OK, I think I've hit an insurmountable opportunity!"