Hi Margaret,
Whatever car you drive, fuel consumption in France will roughly be the same as in the UK - except for:
1. The French motorways / autoroutes are generally much less busy than any equivalent UK motorway, so it's much easier to maintain a constant speed - and that should improve fuel consumption
2. The speed limit for towing in France is the same as the posted speed limit for the road - so in good weather conditions - you could in theory tow at 130 kph (81 mph).
But at those speeds, the car will drink fuel, and I wouldn't expect the towing experience to be that nice - so if you stick to UK speeds - fuel consumption won't be that different.
3. Other costs - if you drive from Calais to Normandy, then it's possible to travel by autoroute and then pay the toll fees where applicable, or use the free N roads and sections of toll free autoroutes - which is also a bit slower.
As a rule of thumb, the sections of toll autoroute will add a cost of around 1/3 or more to the fuel cost of the journey
Use a web-site like viamichelin to plan and calculate the actual costs - this has an option to enter your type of car, your caravan, and typical fuel costs...
Robert