The 50A fuse below the sink is the main house fuse from the battery under the step. If its blown you'll get no 12V from that battery (when unplugged on "use").
The kitchen drain should go below the floor and across the coach to the grey tank. If you go to Thor's website on the contact us page you can request drawings for all of the systems on the coach; at least you can see how it should be hooked up.
Here:
https://thormotorcoach.com/contact/ click on the "Customer Care" link at the left.
Thor Forums
