In metric units, I would expect fuel consumption of about 20 litres/100 km without a trailer, about 25-30 litres/100 km with the trailer.
How much do you pay for gasoline in France? 2.50 Euros per litre?
Almost any Chevrolet 1500
(which will give better economy with a 5.7 engine) has the capability of towing the Safari; maximum weight of the trailer is only about 2500 kg. The real issue is tongue weight, which could be as much as 250 kg. If there is no weight-distributing hitch with the trailer, can you buy one in Europe? If you can't, the 3500 dually would be able to carry the tonque weight on the hitch. It's certainly the easy way for you. However, the ride of such a heavy truck may be too rough for the long term good of the trailer.
Does the truck have an electric brake controller for the trailer?
If you haven't done the research already, you will have a number of questions to resolve. What EU regulations concerning brakes and hitches do you have to meet? Will the truck and trailer meet them?