Hi SMD and welcome to the forums.
Unfortunately, towing has a massive detrimental effect on MPG, there is no escaping this simple fact.
A caravan isn't the most aerodynamic of beasts and even the lightest caravan will reduce MPG due to wind resistance. The heavier the caravan the harder the towing vehicle has to work to maintain a reasonable road speed during the journey.
There are also variables to take into consideration such as:
- the time of year, petrol and diesel vehicles are more economical in warm weather.
- the weather, high winds can be good or bad depending if you are driving into the wind or it's pushing you along. Heavy rain, fog and snow will all affect your driving style and therefore mpg
- the terain, hills are a killer because you have to put your foot down more to sustain speed
- stop start traffic where you are using the lower gears. Motorways and dual carriageways are by far the best bet for higher mpg because speed can be maintained over greater distances, in theory.
- the performance of your vehicle, a more powerful vehicle is going to be more economical than one where lower gears have to be used to maintain progress
- type of fuel, quite subjective but before Mr Putin went berserk I was using Shell V-Power diesel and I found I was getting much better MPG that it justified the extra cost at the pump. However, I think I might be returning to supermarket jungle juice now though, thank you Vladimir...not!!
- the speed you choose to travel at. When I first started caravanning, I wanted to hit the speed limit every time. I didn't want to be the shed dragger holding everyone up but it comes at a cost. I've realised something as I enter my 63rd year, "the older I get, the faster I was". I'm now happy to follow the heavies who are on their limiter at 56mph on the motorway. Remarkably, this simple trick has increased my MPG on the motorway from around 24mpg @ 60mph to around 30mpg @ 56mph. On one occasion through a quiet section of roadworks on the M1 with a limit of 40mph my MPG climbed to 38. So, the need for speed while towing is going to cost you dearly.
This is by no means an exhaustive list but I hope it helps answer your question.