electrical problem- where to go from here

everything on the dash is dead. turn the key-nothing. no gages, the chassis battery has about 12v. the radio doesn't work and i believe that works off the house battery. house batt is 13.5v

it is a coachmen mirada 2003. the coachmen tech disavows any knowlege since they merged with forest river they know nothing about old rv's thanks for any input