Hi everyone-- thanks again for all the very helpful ideas you shared here-- very much appreciated.

We've finally solved it: someone at some point installed oxygen free wires (like for a car stereo) between the leisure battery and the distribution panel. As these wires do, they'd oxidized and were no longer transmitting the electricity well-- only a bit getting through so the system had isolated the leisure battery from the rest of the system. We cut off the oxidized portion of the wiring and are back up and running as normal.

Thanks everyone!