When you have everything hooked up use a volt meter at the converter terminals at the power center then check the voltage at the battery terminals.
You will see a difference in voltage because of losses in the wire. Resistance is related to wire size and length. When your batteries are low and you do that voltage test you will see the most difference in voltage readings. As the battery charge rises you will see less of a difference in voltage.
This isn't so important if you are going home to plug in or did some dry camping and are then heading to a campground to plug in. If you are running on generator or solar you want as much of the gasoline or sunshine to go back into the batteries asap. Many OEM converters only put out 13.6 volts and it's normal for 13.5 or 13.4 to get to the battery. Upgraded converters will hit 14.6 to 14.8.