Hi Guys,
Having the same issue on my own coach. The lugs the manufacturer used when fabricating the cables used "open" end lugs which exposes the cable ends to corrosion. Over time the corrosion migrates up into the copper strands inside the insulation. Cleaning the connections does not fix this condition. Undo resistance from this condition raises havoc with the inverter/converter which mis-interprets the true state of charge and battery condition. The original big black Freedom 20 charger was notorious for boiling batteries because of dirty battery terminals.
When replacing the lugs, use "close ended" cable terminals. Before installing the terminals, slide a piece of heavy duty heat shrink tubing over the cable then crimp the lug. Position the heat shrink over the shank of the lug then use a heat gun or torch to seal the lug to the cable. Corrosion is limited to the connection only. Use anti-corrosion felt pads under the terminals and the red sealant spray on terminals. Hope this helps, Fred