Many Class A's have a 'partial' master shutoff switch that only works on some items.
Other items can still draw from the batteries. Like the memories of the engine, tranni, and radio, along with the safety devices like the Propane, CO detectors. Then there is the step circuit that is always ready for operation, even with the storage switch 'Off'. If you can set the storage switch to off, open the door and the steps extend, then not all loads are disconnected from the batteries.
Why they do this sort of thing is obvious for the most part...the engine and tranni memories are there to 'learn' your driving patterns so every time you start the engine, it's ready to behave like you expect. Tranni memory is for the same reason. The radio memory is so you don't have to search for and save all your favorite stations repeatedly. The safety devices often are directly wired for obvious safety reasons...the fewer connections between the safety device and power the better. Same with the steps (people have stepped out when they don't extend and were badly hurt or even died), you don't want someone shutting off the storage switch with the door closed and then stepping out into the void...
And of course, there's always the economic reason the manufacturers often invoke..."we can save a couple bucks if we just wire this circuit directly to the battery main with a fuse! Less wire!"
About the only way to be sure, is to actually measure the current drawn from the battery when the storage switch is 'Off'. I could do that, but since just disconnecting the battery grounds is so much easier, that's all I do. Others like the switch. Years ago it was hard finding a switch rated high enough to work correctly without going bad too soon from overload. Now I see they're either made heavier duty, or they just changed their ad copy.
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