Voltage loss with key in run position, engine not running

The first step is to measure the battery voltage with a digital voltmeter. Dash gauges are notoriously inaccurate. The best they are good for is for YOU to recognize when something is not normal. Measure the voltage right on the battery posts, not the cable clamps. A good, fully-charged battery will measure 12.6 volts. If you find closer to 12.2 volts, it's okay but discharged and we'll need to test the charging system. Charge the battery at a slow rate for an hour, then see how the starter cranks the engine.

If battery voltage is okay, check it again while a helper tries to crank the engine. If the voltage drops lower than 9.6 volts, charge the battery, then try it again. If the voltage stays up, move the meter probes to the cable clamps and take the measurements again. If they are lower now, there's a loose or corroded connection on one of them.