Not ever seen your setup I would think the TRANSFORMER is only a step UP unit to get the US 120VAC up to the Aussie 240VAC for your trailer needs. Not sure what that would have to do with switching in your 12VDC items...
Your 12VDC is generated by both the on-board converter/charger and/or your battery setup. In your case the converter/charger unit operates from 240VAC.
The whole thing should look something like simplified block diagram except all of the appliances are 240V verses 120V as shown. The 120V to 240V transformer will be inserted at the very beginning where the SURGE PROTECTION is installed.???
If the transformer is working normal you should be able to read 240VAC at all your outlets. The 240VAC must also be present feeding the converter/charger unit before it will develop your 12VDC. As you can see both the converter/charger unit and the on-batteries feed the 12VDC Power Distribution Panel to supply 12VDC to all the internal points of the trailer as needed.
I suspect something has tripped on the 240VAC side of the transformer. I have no idea if any of that is controlled by circuit breaker, fuse, or auto-reset circuit breaker or what. The other item that have tripped out would be the surge protector device if you are using one. This unit does have a reset mechanism. Might want to look at that as well...
First thing I would look for is 240VAC at the outlets inside the trailer. If that is missing then go back towards the transformer from there. To me it almost sounds like the MAIN trailer 240VAC 30AMP circuit breaker has tripped. I would manual reset all of the 240VAC circuit breakers in the panel. i.e. turn each one OFF and pull down as far as the lever goes and then back up to ON. You should feel it reset if it was tripped...
Might pass along what Power Distribution Panel you are using - This is the 240VAC panel where all of the circuit breakers are located on one side and the 12VDC fuses on the other side...
Once you determine the converter/charger has 240VAC on its input then you can start looking where the 12VDC is not working. The surge of turning the transformer ON and OFF may have blown a 12VDC fuse as well. The REVERSE POLARITY fuses are suspect as they are in-line with the 12VDC OUTPUT.
Just my thoughts - haha...
Was you able to get in touch with Robert Ryan on the RV NET. He has been our key advisor for all things Australia Trailers for years.
For all I know about the JAYCO EXPORT TRAILERS maybe they all still use 120VAC APPLIANCES as we do but a 240VAC to 120VAC step down transformer is used so you can operate the unit in Austrailia???? Then when it is is used here in the states you just bypass the transformer with relays???
Are you here in the states with your trailer or in Australia.
Roy Ken