Sounds like a blown fuse for the battery disconnect circuit based on the what isn't working and DC volts at the disconnect terminal. If necessary you can use a jumper cables or other heavy wire on the solenoid terminals to bypass the disconnect and get the house systems working enough to get the coach to a service center. Once you get the coach to a drivable state best to remove the jumper.
But before you jump the circuit triple check your battery configuration with the two 6VDC batteries. You really need a meter rather than a circuit tester to be absolutely sure. If somehow you have them connected wrong and send incorrect power to the house equipment (fridge, furnace, water pump and such) you're likely to allow a lot of magic smoke out of various appliances. Getting the smoke back in can be expensive. A bit like plugging the couch into a stick and brick electric dryer outlet for AC power. Works great for about 10 seconds.
Ensure the door is closed and the step switch is off so the steps will retract before jumping the circuit.