Pretty simple stuff. Can you use a meter? What is the voltage on the house battery? What is the voltage on the 12 volt circuit panel?
Does it work when you jumper the two batteries? Does it work when you only jumper one side?
Assuming a good battery in the house, jumper from the house battery to the 12 volt panel, first do the negative side. Does it work now? Try the positive side? Try both together?
Something is disconnected or loose. That should find it.
Mike
2012 Winnebago Impulse Silver 26QP
2005 16.6 Double Eagle
2018 Jeep Wrangler JK
previously Snowbird Campers,
Triple E Motorhome and Fifth Wheel