You do need to quantify this complaint. What voltage does it read on startup and what does it drop to?
Also, I suggest a test.
Put a multimeter, in volt meter mode, on your battery, at rest, to see it is at 12.6V. Then have a helper start the truck. See what multimeter does. (state actual values).
Depending on what it does and the recommended test/causes others have advised, you will eventually find it. A voltmeter will start high and drop also.
Some causes, IF there is a problem.
1. Bad Gauge
2. Bad battery
3. Loss of ground somewhere
4. Bad alternator
5. Cables/post clamps
6. Idle speed
7. Trailer lights?
