11 volts with the engine off - key on, or motor running at idle? Are there any aftermarket accessories, such as electric fans, electric fuel pump, high powered stereo with amplifiers, aftermarket ignition system, fuel injection, etc. attached to the vehicle? Where are you "measuring" your voltage from, at the battery, the alternator, or an aftermarket gauge wired into the dash?
Add-ons can suck a battery quick if the alternator isn't up to the task. I have most of the accessories listed above and had to go with a 130 amp alternator for mine. With the key on - motor off, it's 11 volts normally. Running down the road at 80mph/3k on the tach it pushes almost 16 (borderline overvolt).
Have you bypassed the ammeter yet? Is the wiring factory original? Those bulkhead connectors were a weak link and can cause issues, melt, even create an electrical fire, especially with high draw add-ons installed. Also, have you run a wire from the alternator post directly to the battery yet? This provides a direct link around the in dash ammeter to charge the battery, it may render the ammeter useless but charge up the battery directly with priority. Go heavy gauge if you haven't, I went with a #2 gauge battery cable on mine.