[triffic]

In general, the "lesser" Sargent controllers such as the EC155 (and EC325 and EC328) only allow you to charge one battery at a time - so the battery that's selected on the panel is the one that is charged, and the one that power is taken from by loads in the habitation area. The default on the EC325/328 when the panel is turned off is to charge the leisure battery, but I don't know if that is true of the EC155.

The "low battery" on the radio is entirely a function of the radio - some OEM radios are set up very conservatively and will auto switch off when the battery is still only partially depleted. On the other hand, your vehicle battery may be knackered!

 

[triffic]

My understanding is that the 240v sockets are disconnected until the red switch is set to on

No - you should be Ok there. You should be able to turn off the 12V supply and not affect the mains sockets at all.