If you are reading 11V anytime the engine is running, then you have a charging problem. The ECU, ignition and pump do not draw that much juice. I don't think the batteries are your issue.

It sounds like there is something wonky with your regulator. Tightening the belt and wiggling the plug are not diagnostic activities. You need to either take the vehicle to a place that can test your charging system in situ or pull the alternator and take it to an electrical shop to be tested.

Good Luck,
Michael