Well, I fitted a 120Ah Leoch Leisure Battery on a T-Max split charge and after four days running the fridge with no charge and it was still happy at 12V. It only dropped below 12.2V a few hours before we packed up.
I wouldn't have expected that kind of result unless you have a large solar panel connected also?
I have a proper camper/motorhome 3 way fridge, it automatically detects and selects the appropriate power source, 12V when the engine is running, will switch to LPG when the ignition is turned off, hook up a lead on site and it switches to 240V, I love it, great bit of kit.
I know that some motorhome manufactures are fitting compressor fridges, personally I don't get that especially if it's a small camper, the noise when it kicks in during the night etc.