Hi
I have a BMETs fitted RB44 with three sets connected. The four batteries (connected as Iain says) run to a main fuse panel which has take offs for the penthouse light, cabin fan and lighting and which feeds a second panel which has mainly the 2 pin power sockets seen on Clansman and which are protected by press button circuit breakers. I take the set/control harness feeds from these.
There is no direct feed from outside to the radios - the batteries can be charged via the 24 volt alternator fitted in addition to the 12 volt vehicle alternator or by an external 28 volt generator. I assume that this is to maintain a clean supply to the radios or to remove the need for a pure sine wave inverter within the system. My vehicle was pretty complete when I bought it and the racking and wiring were still in place, and there was nothing there to suggest that anything was powered at anything other that 24/28 volts.
Before I fitted the 353s to the truck, I tried to test them by connecting the power lead directly to a 24 volt battery charger. This failed miserably as the built in trip in the radio kicked in almost immediately. However, connecting two 12 volt batteries between the charger and the sets allowed them to power up perfectly.
I'm looking at mounting a laptop in the vehicle, which despite being a military set, is mains powered so I will need an inverter at some stage, although in my case to convert 24 volt DC to 240 volt AC.
Hope this helps
Richard