Battery charging while traveling

William,

30 amps sounds a little high to me. I would have thought a 3 way fridge on 12v setting would draw more like 14 amps or thereabouts.

Not to worry though. The alternator will put out sufficient current to handle both the auxiliary battery in the back of the ute and the feed to the fridge.

Just on that however, the fridge should not be running off the caravan battery. The battery is for powering lights, pump, etc. and the fridge circuit should be wired "direct" to the primary (or auxiliary) battery circuit via a heavy duty cable and Anderson connectors.
When the ignition is turned off, no current should flow through to the caravan fridge. The circuit should be "disabled" to stop flattening the battery when the engine is not running.
The 12 volt setting is only to maintain the internal temperature of the fridge while traveling, otherwise you should use the 240 v or gas mode when stationary.

A compressor fridge however, is a different proposition and can be wired direct to a caravan's on-board battery, which in turned should be charged by the alternator while driving.

In your case, you should have two circuits to the caravan.
One to supply power to the fridge from the vehicle's primary battery and controlled by the ignition circuit. (This is often wired via the 7 or 12 pin plug/socket.)
A separate circuit, using heavy duty cabling (6 B&S is good) and Anderson connectors to the caravan's battery, from the vehicle's primary battery, or the auxiliary battery. Best practice is to have a dc-dc charger just before the caravan battery bank to overcome any voltage drop in the circuit.
The Primary and Auxiliary batteries should be controlled by a dual battery isolator to eliminate any risk of draining the primary (starting) battery.

I'm not an eleco but are you sure that's right 30amps that's a lot of power it must be a very old and large fridge did he check it with a meter to get that number or get it off a plate or sticker on the fridge with the power consumption info either way your van battery won't support it for very long which is the usual story for a 3 way fridge your's just seems a bit power hungry. It also depends on the size of the alternator and that you stay a reasonable amount below it's output capacity or you will have problems. You need to add up how many watts your using when you're towing headlights ,tail ,parkers, blinkers, brake lights when applied, dash.,stereo both fridges everything you can think of then divide the watts by 12 to give you how many amps your using. Then you have to allow for any computers that the cars runs which who know how much that is plus your charging 3 batteries as well the distances you usually drive as well because it take quite a few hours to charge a battery. You might need a larger alternator in the long run.

Count me as another with serious doubts about the 30A .... that equates to 360W @ 12V which is highly unlikely.

An absorption fridge draws about 20 amps (give or take a few amps) when on 12 volt and your Weaco draws about 5 amps when running so 30 amps would be a good figure as it is always better to be over then under.

Shouldn't hurt the alternator but if running lights, heater, AC and other accessories you will start getting up to the upper end of the alternators capacity and duty cycle and not forgetting your charging 3 batteries as well.