Is there a MPG rule of thumb when towing?

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Really, AD has said it all and I’d not be willing to guess at any figures with so many variables in the mix.

Just fill your tank and see how it goes.

From my experience I would suggest that the towing MPG will equate to around the "around town figure" suggested by manufacturers. I am not sure you can tow a caravan and expect decent MPG figures? What you have to ask yourself is how many miles will you tow in a year and will the increase in fuel used really impact on your decision to tow a caravan? Personally I would be inclined to go for a car that is capable of towing the caravan competently rather than worry about the economy.

David

With my last tow car, a 1.8ltr diesel Ford Focus. The caravan increased the total volume by 3 and the total weight by 2. I was therefore happy to get half the mpg of solo driving. Around 30 mpg. My present tow car, a 2.0ltr diesel Ford Kuga gives me between 28 mpg and 32 mpg when towing depending on how fast I'm driving. 50 mph roads are noticeably  more economic than 60 mph motorways. I now have a much better weight ratio with the Kuga, but it is at the expense of much poorer mpg when solo.

Land Rover Freelander 2 towing a Bailey Unicorn Valencia S3. Average towing mpg is around 25 or maybe a little more. The Freelander is not the most economical at the best of times but is an excellent tugger!

Much will depend on the type of roads. A long motorway drive will probably be more economical than A roads when you will be in traffic and up and down through the gearbox.

Too many variables to give an answer. However I would echo David's comments. Identify a car that tows well and that you like driving solo, which is what you will do most of the time, and just accept that you will probably get only high 20's low 30's mpg when towing.

Fuel economy in general depends a great deal on how heavy your right foot is. I CAN get 30mpg around town from my ML350 but equally I can reduce that to the very low 20s. When towing my large twin axle I get around 20mpg.

Likewise with my Touareg towing a 2000kg van. You can also expect increased tyre and brake pad wear, but it's all part of the cost of owning a caravan.

It's been a little while since we towed, though we did it for many years, but if pressed for a "rule of thumb" as asked for by the OP, I would suggest to expect an mpg of  slightly more than half what you would normally expect on a long run without a caravan. Therefore your quoted 50 mpg vehicle in the original post may return around 28 - 30 mpg when towing. 

Dragging a heavy aluminium non aerodynamic box around the country will reduce mpg significantly. I used to find I sometimes needed to select a gear at least one lower than normal just to maintain a decent speed on even the slightest incline, and in the hills a much lower gear. If there is also a head wind that can have an impact on maintaining the required speed, the gear to choose, and ultimately fuel consumption.

As others have mentioned it will depend very much on the type of driving. Our 2014 XTrail towing our Bailey Unicorn Cadiz, returned about 28 when towing on standard roads with a 50 mph speed limit, closer to 25 mpg on the motorway at 60 mph. Weather also has a big impact, towing to Baltic Wharf into a strong south westerly, I was surprised to see I had only got 22 mpg. Coincidentally the following weekend was also windy and we managed 32 mpg on the way back.