when at idle the volt meter drops to about 15, but when increased rpm it shows a

Sounds like the Alternator is over charging your battery, the voltage regulator inside is not keeping the voltage at the correct voltage. It may have already damaged your battery as well, take it to your local parts store like Advance, AutoZone, O'Reilly, they will test both parts for you free of charge.

Don't trust the voltmeter on the car alone. Get a real meter on it and see what is going on. Your truck should be charging between 13.9v and 14.7v at idle and at 2000 rpm.