1978 Chevy van G20. No voltage to ingnition switch or lights

GM likes to tap off the large starter terminal with fuse link wires that feed other systems. Start by looking there and tugging on them. If they act like a wire, they're okay. If they act like a rubber band, they're burned open. More commonly you'll find a wire corroded off. The best repair technique if a wire is broken right by the terminal is to install a new crimp-on style terminal, solder it too for the best connection, then seal it with a piece of heat-shrink tubing.

Doc, you got this thing covered on GM for life

Sounds like you're referring to the bulkhead connector. That is sealed to prevent moisture entry. The terminals have a brownish-colored grease to help keep them from corroding.

You might check that bulkhead connector by wiggling it, but your description really sounds like what happens with simple loose or dirty battery cable connections. After sitting a little while, they will often make sufficient contact to pass enough current to run a few lights or the radio, but the connection will be broken from the stress of higher current demands as in running the starter.

To find the break, when the problem is occurring, use a test light or voltmeter and start right at the battery terminals and measure the voltage. The readings will be more accurate if you leave the head lights turned on, even if they don't work. If you find full 12 volts, move the negative lead to the body, then the positive lead to the starter. You're looking for the first place where voltage is lost. When you find the first point where voltage is missing, you just passed over the break in the circuit.