Help! Lance 850 isn’t getting power from truck battery

Check for reverse polarity. Connect a long wire to the truck frame and measure the voltage in the camper. Ground should be 0V and positive 12V.

Then using the same ground connection turn on lights and track down the 12V disconnect. l'd double check the ground again. A voltmeter will read OK with a high resistance connecton without a load because the voltmeter draws virtually no load.