It makes sense from a power-management standpoint to replace both the propane detector and the CO detector at the same time, since newer units are available with considerably lower current draw.
I actually use West Marine for most of my SMB purchases: switches, wiring, fiberglass wax, etc. Marine quality gear seems to hand in hand with our van. I'm lucky, I have one about 2 miles away.
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2007 Ford RB Diesel SMB 4x4 Pueblo Gold; Custom configuration (aisle layout); PIAA 580 driving lights; Picked up on Oct 19, 2007.