This modification may need your garage or auto-electrician to sort but it’s a simple, straightforward modification that could save you from a flat engine battery.
Ask ‘your man’ to swap the cigar lighter feeds from the engine (starter/base vehicle) battery to the habitation battery(ies). This now means that you will have constant habitation battery 12 volt sources in the cab, with the ability to gaily plug in whatever 12 volt device you want and not run the risk of flattening the starter battery. Nice, when you’re maybe parked up for a few days, not on mains electricity, and want to charge up or run various devices in the cab – peace of mind!
While he’s about it, maybe he could wire the internal cab light to the habitation battery for you? This would mean, again, that your engine battery is not used by this handy light and you could perhaps make more use of it when you settle down to read in the evening?