He’s just this guy, you know?
You’d probably want to use a couple of SPST relays or a single DPDT relay for that, with the main power connected to the normally closed circuit (center pin) and the house power on the normally open circuit. A rocker switch could then serve double duty to activate the main stereo power (yellow wire) relay and activate the acc line (red wire) on the stereo. Run power to the switch and relay from the house battery and away you go.I'd be interested in more details on this. I'd like to power my stereo and amp (both aftermarket) from my house batteries for camping. Maybe not quite the same setup - maybe some type of switch where the stereo/amp draws power from main electrical when the van is running and then a switch to draw from house when not.
Alternatively, I’ve been playing with the idea of installing a house battery and putting in a battery selector switch to run everything without running a crazy amount of extra wires for accessories. Someone on here did that, I think Aquaputana. Some of them even have an option to connect both batteries which could be helpful on really cold mornings when she’s cranking slow