I too have the 30a sterling battery to battery charger. Mine charges a 24v leisure battery in the box. 90% of the low voltage DC electrical equipment I am installing is 24v, the only exceptions being the thetford toilet flush valve, the waste-water dump valve, the piezo ignition on the cooker, and the fan in the cooker hood. 24v to 12v voltage droppers are cheap and simple, I don;t know why your spark thought they were unreliable. Viictron and the like do nice ones, but even the cheapies will do fine for low power stuff. Even if they do break, you can buy them anywhere and they will be cheap.
24v to 12v droppers do not work well to charge 12v batteries from a 24v supply.