House (coach) battery is dead, as Mandalay Parr, suggested, or the cables connecting it to the rest of the world are not making a proper connection, or there's a main DC fuse near the battery that's blown. Go over all the battery connections carefully, especially including the ground connections. I suggest tugging on the cables rather than just staring at them, as I've once or twice seen battery cables that are internally corroded clear through but look OK from the outside, at least at first glance, as the insulating jacket is A-OK.
The solenoid between the batteries is to enable the house battery to be charged from the engine alternator when underway, and possibly to enable the "emergency start" feature if you have that in case the chassis battery gets discharged.
