Fridge may have a low voltage automatic cut out. So firstly check the the fridge hand book for voltage cutout and then check voltage to the fridge at the fridge end not the battery.

Check that your Anderson plug is still connected to 12volt from the car. It may have been disconnected when the ctek charger was wired in.
Where is the anderson battery getting its power from, is it the spare battery in the car that you now charge with the ctek ?
I am assuming your fridge is an compressor type fridge ( 3 way fridges use a lot of power on 12 volts )
Perhaps a little more information on the fridge and your battery set up would help...you did say the fridge isn't connected to the van batt ( is the van batt connected to the ctek charger and not the car as I assumed )
check your Anderson plug fuse / breaker and batt connection.

Also check voltage at plug .

Just a query, you had the fridge running off the 12v supply from your tow vehicle, and everything was fine, then you had a DC/DC charger fitted to the vehicle. So do you run the fridge via an aux battery fitted to the tow vehicle, have you just had an aux battery fitted to your vehicle, and previous was the fridge wired directly to the vehicle start battery, or is the dc/dc charger wired to charge the caravan house batteries from the tow vehicle and the fridge is still charged from the tow vehicle.