You need to be checking voltage with a voltmeter in the airplane.
sounds like you have something in the circuit providing more resistance than you want.

Power up the aircraft on this battery and measure the voltage at the battery, it should be up at in the 12s like you expect. if it is not then you need to measure the current being pulled and possibly test with another battery. to see why it is different than you bench testing. Are you pulling more current than your bench testing? What devices are pulling that current?

If it is up in the 12s then measure at your onboard voltmeter and confirm it is reading correctly. You said it has been reading about 11.4. You should not be losing a full volt between the battery and your voltmeter. Then start checking the voltage before and after each component between the battery and your volt meter to see what the offending component or wire is. You may find it is a combination of wires or components that add up to the total voltage loss you are seeing.

Brian