WFCO units have many negative posts related to charging, don't recall a GFCI issue. Before you purchased the second board I was about to ask and/or suggest considering a good replacement based on charging considerations.
With the charger off and using a decent 3 stage charger (even a portable auto charger) plugged into the rig does the GFCI trip?
Have you checked what other plugs are on your home (not rig) GFCI circuit? Anything downstream from the plug to the rig panel are possible problem areas.
