You need to get your rig level in order for the propane fridge to work. They work by a gravity feed of the material inside. Bascially, a burner heats up the liquid, which forms into a gas, and then condenses and runs down an intricate series of tubes back to the bottom, absorbing heat as it does so. If your rig is not completely level, then the liquid does not run down evenly, and you will burn out your fridge. Then your beer goes warm and the fun has definitely gone away.

Modern RV's have hydraulic/electric 'feet' that extend down and both stablise and level out the rig. These take time to put up and down and are very obvious when the are extended. If you just park on a couple of bits of wood, it's possible to get it level enough for the fridge to work OK. It doesn't look as obviously 'parked' to the casual passerby. And if you need to make a speedy getaway (for whatever reason, be it the law or unwanted company) you can just start up and drive off. That's what the wood is for.