good point well made..
However… (you knew that was coming didn’t you?!)
The vehicle I had was a Bedford CF 1973. Simple charging circuits, simple electrics, nowt fancy here don’t ya know?
The switches were never operated while driving of course, only while parked up for camping, fuel, eating, drinking, sha… erm no.
The batteries couldn’t drain into the other, (a valid concern) because neither was ever that flat,(the point of the exercise) and both batteries were only connected when the engine was running, Therefore the alternator was charging both batteries and equalisation was minimal.
There was always the vehicle battery connected when driving, (so no damage to teh alternator)the leisure battery was switched in and out as required to charge it. Now in theory, rapid charging from an alternator COULD damage the leisure battery.
But the amount of time it was ever connected, and the fact that there was another load on the circuit while driving, I never suffered any damage. I had the same battery for 5 years with no problems.
I used to sell batteries for a living, and know all about the effects of over charging, under charging, dead loading, etc. But in my experience, with a simple vehicle, and common sense, it worked. And worked well.
The whole point was avoid using diodes and fancy electrickery stuff. And in my experience, electronics fail. Often. Big chunky switches don’t.