If you unplug your camper so the converter is not powered, check the voltage of your batteries at the battery terminals where the converter/charger connects.
If you are about 6v low, you have a wiring problem with your bank of 4 batteries. If it reads 11.6v-13v, it is wired correctly to the converter/charger.
If wired correctly, the ONLY possibility is that one battery is defective.
Second method is comparing battery to this chart for 6v batteries:
5.4 volts is NOT a state of charge for a properly functioning 6 volt battery. Print out the chart and take it with you. If they argue, ask for a manager. Perhaps mention how you expect at least a working knowledge of how to test and evaluate batteries and that recommending continued charging and use of a defective battery that has failed internally could pose a risk of damage and possibly even danger to the user and his property.
Then ask, "Why are you refusing to honor the warranty on a defective battery? Please put your reasons in writing, including why you believe 5.4 volts is an acceptable level of charge for a 6 volt battery when all the documentation shows otherwise."
What people will say verbally is too often a far cry from what they are willing to back up in writing.
Guaranteed results ;)