The relay only operates when you plug your rig into shore power. The relay closes off your house battery and allows the battery substitution unit to power everything in the coach. When the rig is disconnected from shore power the relay clicks back to a different set of contact points, and your rig will then be operating off of your house battery once again. Shore power is transformer power, house power is battery power, as far as your lights, heater, and pumps are concerned. This is a primary reason that attempts to recharge a flat house battery using an on board generator, fail miserably.
Flat batteries require a minimum of thirty to forty amperes of power to recharge quickly.
There's more that I haven't told you about. Switch on a couple of lights on inside the coach. Unplug your shore power. Older converters sometimes fuse the battery separate from the converter. So check all of the fuses.