Not difficult to answer this!
Firstly, if you want to spend less on fuel, drop your speed to average about 90km/hr. When going uphill be prepared to lose 10km/hr and on the downhill use the slope to get to 100km/hr. Try not to ‘floor’ the accelerator on the inclines rather shift down to keep the engine revs in the band where you have a reasonable torque output which for most cars is around 3000rpm (petrol) or 2200rpm (diesel).
Secondly, aerodynamics play a huge role and there is very little you can do about it because its the caravan design that determines how air flows over the ‘van. Make sure that there are no protruding objects that interrupt airflow like side mount awnings etc. If you have an open A-frame mount a flat sheet over the top (bottom is even better) to help smooth out the airflow below the vehicle combination. Also, the stone protector that is so popular on caravans adds to the drag…decide whether you want to risk getting some stone chips on a gravel road or save some money on fuel.
Thirdly, check your brakes and wheel bearings for rolling resistance…reset or replace to improve/reduce rolling resistance.
Finally, a more powerful tow vehicle will show less of an increase in fuel consumption when towing BUT it will use more fuel in normal day to day driving e.g. a 3.o litre V6 will maybe average 16l/100km for day to day driving which may then increase to 18l/100km when towing a smallish caravan however, a 1.6l 4-cylinder may use 8l/100km for normal driving increasing to maybe 14 or 15l/100km when towing the same caravan mentioned above. The shock is the percentage increase that you have to deal with. On the other hand, the 3.0l V6 will last many years longer if you tow on a regular basis.
Hope this helps…best of luck with the car search.