The fridge draws power from the leisure battery. So:
- are your leisure batteries fully charged before you plug the fridge?
- what’s the power consumption of your particular model?
You have two 75amp leisure batteries in your Ocean. So 150amp in total when fully charged. But in practice, only half of that is usable as your batteries will get damaged if you get them to less than 50% capacity. So assuming new, fully-charged batteries and nothing else using electricity in the van (I.e. your don’t have the Ocean’s built-in fridge running, or the diesel heater running or anything else plugged in), you have. 75 amps for your fridge-freezer.
The Brass Monkey website doesn’t seem to have any tech specs. But this random website on the internet seems to say that the 15 liter model draws 3.8 amp (I assume per hour):
https://www.roadtechmarine.com.au/15l-brass-monkey-portable-fridge-freezer/p/GH1623
So that would in theory give you 19 hours of running your freezer before your leisure batteries get into danger territory.
Now, that’s not necessarily how long you’ll get though.
- if you have a larger model, it’ll draw more current so your batteries will deplete faster
- if your batteries aren’t fully charged (I.e. plugged at the mains for at least 24 hours), you’ll get less
- if your batteries are old, they capacity may be lower
- if anything else is using electricity in the van, you’ll get less runtime
Generally speaking, these things use up a lot of electricity to get to temperature and then use a lot less electricity to maintain the temperature. So it would help to get the freezer to temperature by plugging it to the mains (or having your California plugged to the mains) and then using the leisure batteries to keep it cold.