The dash will not show you the voltage of the auxiliary battery that ESS uses, only the voltage of the main battery. Even if it did, you don't seem to understand that you would have to check it with the engine OFF, because when the engine is running, the batteries are charging (and will always show 14.x volts). You can't read actual battery voltage with the engine running.
They use super cheap, bargain bin batteries that are marginal when they are brand new and then die a slow death sitting on the lots not been driven. You need to either convince the dealer to install a new battery, or go to a shop and pay out of pocket to have a decent auxiliary battery installed (and probably a decent main battery too) or this will just be a continuous issue for you like it is for me. (Both my batteries are still original, both are still weak and give me occasional issues because the voltage is too low, but over a year later I am still too busy to be bothered to address it. Still starts and drives, so whatever.)