Electrics: 850: Volvo 850 GLE T5 Display failure: Rev Counter / Temperature

[mseville]

Has anyone come across this intermittent problem: when a sat nav and a dashcam are together plugged into the cigar socket, it sometimes causes the engine temperature gauge to read zero, the rev counter to jump about all over the place (including hitting full deflection with considerable force), and the automatic gearbox fault to be displayed (orange upwards arrow).
There was no problem this time on the out-journey towing the caravan, but upon return after a few days standing, and with significant load (air conditioning plus the internal fan) this occurred again. Incidentally, the caravan does have a lot of 12volt (non-LED) running lights*.
[1] Does anyone know of a cure without sacrificing one of the driving aids?
[2] *would switching off the running lights help the situation, and if so, how would I do this? (I have wanted to do this for a while, anyway, so that the caravan brake lights coming on would be more apparent).