I'd be focusing on the battery charger.
First thing I would do is plug the charger into the electric and test the voltage output using a multimeter. You should get a reading of around 13.9 to 14.2volts.
If you get nothing then it could be a break in the cable leaving the charger at the gland. I had a break there and had to repair the break.
If you switch the charger on and off at the socket and you manage to start the charger, a capacitor on the circuit board has likely blown.
The charger is brand new spruce.its working fine mate.(numax connect and forget 20amp charger)
Problem is Daz, you don’t know if the charger is working correctly unless you follow what spruce has said. It could be lighting up faulty and saying the battery’s are fully charged when they are not.
If you don’t have a multimeter, check your batteries voltage with the engine off using the controller. Then plug the charger in and you should see the voltage increase. If it doesn’t, then your charger isn’t working properly.
There’s no way that your batteries are fully charged from even an hours driving a day. These heaters take a wack of charge out.