When private trip with 7.5 tonner

I have a question for my colleagues. I have FS III and can drive with 7.5 ton and trailer up to total train weight 11,999 kg. I regularly drive to Italy to Trani (40 km from Bari) to indulge in marble around my hobby. A route from Vienna to Trani approx. 1,400 km. If I load the Tandemach trailer on the truck on the way to Trani this trip could be declared as a private trip, which is what it is, because I bring the marble for my purposes with my name. I certainly can’t go back, because I am over 7.5 tons with tandem trailers. I’m looking forward to your Meining.