Hi
@Chris Daly,
welcome to the forum.
You have asked an important question.
I can explain what we do.
Firstly, regarding running the fridge on 240V to cool it down before you head off.
In the cooler months, I usually only put it on 240V first thing on the morning before we leave. That means it is on for 2-3 hours.
In the hotter months, I might put it on 240V last thing before going to bed at night.
Regarding running your fridge on 12V. This topic comes up quite a lot and if you search through the forum you will find it has been discussed many times.
It is definitely not recommended to run your fridge off the battery in the van (house battery). On 12V your 3-way fridge will draw between 15A to over 20A depending on the rating of the 12V heating element in your fridge. My 90l Dometic RM2350 has a 175W element and draws 15A. Other fridges have larger wattage heating elements.
A typical house battery has a capacity of 100Ah. The fridge will completely flatten the battery in around 5 hours!
On the morning before leaving, I also fire up the fridge on gas to make sure it is actually working. On one occasion we loaded up, drove off and arrived at our destination 6 hours later to find we couldn't start the fridge on gas. Turned out to be a faulty gas regulator.
The way most people work to run the fridge on 12V is to run it from the tug but ONLY when the ignition is on. That means running wires from the battery on the tug back to the towbar but also installing, up near the battery, a fuse or circuit breaker and a relay to disconnect the feed to the fridge when the ignition is off. This is important since you don't want the fridge flattening your tug battery when the engine is not running, such as stopping for an extended lunch, OR forgetting to switch the fridge over to gas or 240V when you arrive.
There are two ways to connect the battery to the tug at the towbar.
1: Use the heavy 35A connectors in the 12-pin plug. This is now the standard way Jayco do it.
2: Many have had problems with the pins in the 12-pin plug heating up and melting the plug. So. a more accepted way is to connect the fridge via a standard Anderson connector.
Many people have issues with running the fridge efficiently on 12V. It is imperative to ensure the size of the wiring is adequate to supply the 15-20A needed by the fridge AND the voltage drop across the cable run is low enough that most of the power gets to the fridge and is not dissipated in the cabling. 8B&S or 8AWG is the minimum. Many people run 6B&S or some even 4B&S.
I hope this helps you get started. Don't hesitate to ask questions if what I have written doesn't make sense or you need further clarification.
The information I have provided above was mostly obtained on this forum over the past 9 months. Back then, I like you, was also new to caravanning. Back then I couldn't even spell caravnner, now I is one!
cheers
Mike