There are some newer ones that will automatically swtich to electric if it detects it's on and will use it rather than the propane, so you have one like that it would need to be disconnected from any 120V or 12V system so it will just run on Propane only.
As for propane useage I'm not sure as it can depend on climate temp and the size of the frige. Now Lehmans sells these in a 10 cubic foot and they also have small 10 cubic foot freezers, they use to list the useage for each propane and K-1 models. You may be looking at 20 pounds a month or less and around here I can get a 20 pound BBQ tank refilled for $13.