I mean aren't on hook up!
I have a cfx 35 & Beach T6,
Always put the van on hook up for 24hrs before a trip and at least once a month, to fully charge the Leisure battery.
I always pre cool my fridge whilst plugged into 240v at home, set fridge on minus 4c , then when -4c achieved, load it up with pre chilled food , drink etc (from the home fridge)
when fully loaded for the trip, reset the 2 or 3c, let it run until the fridge achieves & holds that temp For an hour or so.
When I’m ready to go on the road I transfer the fridge to the van 12v socket and set the fridge to 4c.
my fridge will last four days no probelem (I have never had it cut out due to low battery even in really hot weather. )
The CFX35 has various voltage protection setting to prevent flat batteries, which I have never seen kick in .
if you are going to be going in and out of thd fridge regularly for drinks etc, get a small cooler bag and place those items in it so to prevent / reduce the incidence of opening thd fridge (wine, beer, pop, cooking time etc, etc)