I would start by cleaning the battery terminals. Measure the voltage drop from the center of the plus battery post to center of minus battery post. Then measure from center of Plus to Chassis, voltage reading should be very close to same. Now measure from the Plus cable (near the post) to Chassis. If reading differ by more than a few millivolts, you have bad connections. If the readings are not 12+ volts, charge or replace battery.
Good Luck