You have a truck towing a trailer. It is just to keep the drinks and some food cold while driving till you can get 120v for the fridge.
You can try the inverter route with some difficulty as others have posted.
Another way is just have a "cooler" that could hold about as much as the outside fridge in the trailer does. Cooler powered from the truck's 12v system. A DC-DC converter might be useful in that role, but maybe not necessary. Cooler could be inside the cab or in the box.
"Cooler" could even be another small 120v fridge in the back of the truck box with inverter, where the inverter is powered by the trailer batts on the tongue via fat- wire jumper cables.
Might even be able to use the same fridge by just carrying it back and forth between the truck and the trailer if it is not installed in the trailer in an awkward way.