Long while since I've towed but my big Fourtrak could tow the biggest Ifor with two 17hh in (not that I ever did!). For local running without trailer it did 25 - 28mpg, motorway journeys 30 - 35 mpg (depending on how lead footed I was!) The turbo's make them very greedy on the diesel so avoiding kicking the turbo in kept the consumption down.
Trailers hammer it badly - a 16hh and a 15.2hh in a double trailer knocked the consumption down to 15 - 18mpg. Stop and start running i.e. short journeys, country lanes give poorer consumption.
Contrast that to my big 7.5T Iveco which does around 20mpg local running and up to 24mpg on longer runs with three horses on board. Older vehicles will be worse but it is now financially cheaper to run a smallish horsebox and a small cheap car as a runaround. You can pick up a nice 3.5T that will carry one very large horse or two smaller ones for around £16 - 18K as a new build on a used chassis. No plating, standard servicing, cheaper road tax than a 4x4. Not sure of the mpg on those - Stella2 could probably tell you as she has one.