The cab did have a supply coming from the earthed starter battery. After about 2 years one battery was knackered (not holding a charge, sitting below 12v while the other stayed full), needing £300 to replace both (5.7l diesel needs decent batteries). Perhaps if I'd swapped them around a bit that may have delayed that event if I had known about it before. Therefore I now have droppers!
Some info I've found says the dropper circuit needs to go back to the dropper not vehicle earth, but I can't see anything that suggests that is the case with these ones. The 12v returns go behind a bit of wall with two 24v circuits returns and I can't see what then happens. The sparky who did it is no longer trading, but there isn't a problem with what he did, just what I want to do with it.
Having done the cab feed, taking the earth back to where I think the rest of the 12v dropped circuit goes to, it occasionally blows a fuse.
Sometimes the 8 amp fuse in the "earth" side goes, sometimes the 5 amp in the positive fuse box side. There is mostly 0.2 amps from a rechargeable torch only. I just put a new 5 amp in, and turned on the two cab fans and a plug in worklight thing which will pretty much the most there will be and drew 2.9 max settling to 2.6. No blown fuses. Could I just put bigger fuses in, say 10amp?
Before I found and replaced the blown fuse I measured the voltage in one of the sockets to see if the torch wasn't charging or there wasn't any power at all. At first perhaps 4 volts dissipating down to nothing and staying there. Wait 10 minutes and it'll do the same. Why?!
I could just take the cab feed from the back of a 12v socket the sparky put in since if we're in the cab we won't be using a camper socket, but I'd be happier if I knew what the dropper circuits actually did and how!