Actually the way that solenoid typically works in many coaches, is that the solenoid and two battery banks are disconnected when the coach engine is off. When you turn the engine on and the engine alternator starts producing electricity to recharge the chassis battery, that solenoid engages tieing the two battery banks together. (Some manufacturers engage the solenoid immediately, others after a delay like 10 minutes). Then the alternator is recharging both battery banks at the same time. This is why your 12v house circuits were working without a battery attached. They were being provided power via the engine alternator, to the chassis battery, to that charge solenoid, to your house 12v buss.
The second way to turn on that solenoid is via the red button start boost switch. Remember with the engine off, the solenoid is NOT normally engaged, so you drawing down the house batteries does not deplete the chassis engine start battery. If you hit the boost switch, it will engage that solenoid, and the house batteries in this case will charge back up the chassis battery to provide power to your engine starter. (This scenario assumes the chassis battery voltage is low, but you have enough power in your house battery bank).
You can prove this to yourself by putting a voltmeter on the primary coil of the solenoid (the thinner wires), and watch it get turned on when you start the engine.
Some higher end coaches have alternative approaches, but generically this is a very simple and cheap way to achieve the battery charge and boost functions, so a number of coach manufacturers use it.
DaveB, Raleigh, NC
2015 Tiffin RED 33AA, w/Honda CRV
VMSpc, Magnum BMK/ARC50