In days of old when folks did not want a door to open they bolted a couple of brackets either side of the door and dropped a 2x4 into them and the door stayed closed.
More modern times Camping world sells a device. or two (Dang Found 'em both on the 1st try,, That is rare)
http://www.campingworld.com/shopping/item/refrigerator-travel-locks-latch-style/49699The above may work, I used it on a home freezer where the dang door liked to pop open.
This next one may or may not depending on the design of the fridge.
http://www.campingworld.com/shopping/item/refrigerator-travel-locks-pin-style/49698Finally.. You can attach a small piece of metal to the Fridge, either side of either door, and stretch a Bungee between them for travel.
Or an eye bolt if there is wood trim beside the fridge,, However you need to hold it tight.
