YOu need to measure the voltage at the converter..
This is a line drawing of your system
120 volt---Converter------Long wire with switches and such----Battery
Now electrically
120 volt--Converter-------RESISTOR----Battery
So if the battery is drawing current. THe voltage loss in that RESISTOR (The wire and switches and stuff) will affect your reading
Measure at the converter
IN another post on one of the forums I visit.. An Electronics Type Figured out what part fails in the WFCO's and replaced it. Turns out it's only a few dollars and easily replaced, Plus a few dollars worth of surrounding heat damaged parts.