1979 F250 has no electrical power and won't start

Truck died at a stop light without warning. No lights, radio, etc. Had it towed
home. Replaced alternator and regulator. Checked all fuses and fusible links.
Engine will crank when starter relay is bridged but will not start. Truck was a
camper special and originally equipped with a secondary battery for the
camper. when secondary starter relay is bridged, lights, radio, etc will work,
and engine will crank, but will not start.