I would contact the supplying Garage it sounds more like you have a fuel leak. We have a 67 plate 1.7crdi Automatic Diesel Tucson, live in a hilly/twisty lanes area with lots of short journeys, I would expect our fuel economy to one of the worst around and 11000 miles in we return 37-38 real world MPG so a 90 mile trip would cost £14-£15 so for you to be returning below 20MPG on a trip even with a different engine I would suggest you have a fault of some sort.
when looking at alternatives I would suggest with costs inmind that you concider a Hybrid, if your happy with the Tucson apart from the fuel issue it may be worth flagging the issue with Your dealership and if it is found there is an issue then you maybe able to do an exchange for one of the Mild Hybrid Tucson, that way there would be no early termination charge and your original AP would be considered as Pro rota. The Diesel Hybid Tucson has a claimed 66mpg which equates to around £8 for the 90 mile trip, even allowing for real world MPG as long as it returns early 50’s the trip would come in at under a Tenner. Having the conversation with the dearship first my be a prudent one and beneficial financially to yourself as they may well waver any difference in AP as they get another sale and first refusal on the returned vehicle. Just something to think about.