Even if you run the wires directly to the batteries, when the charger starts you will then read the charging voltage that is going into them, not the battery voltage. You would need to disconnect the batteries from the charger to read their voltage...no way to get around that. I bought one of the 'power port' voltage meters and plugged it into the 12V outlet in the kitchen. All you need to do is find 12V and tie your meter in there. It will show what the batteries are doing - to read only the battery voltage, unplug shore power - its the only way.
Oh - if you want to plug the hole left by the switch, go to your local hardware store and buy a blank plate and install it over the hole.
2002 Holiday Rambler 30RKD
2000 F250 diesel extended cab short bed
2014 Demco Autoslide 18K