Aliner electrical problems - need advice

Assuming the power center is good, the fan running indicates the unit is supplying enough output current that it needs to turn on the fan to provide cooling for the internal electronics. Since with the battery connected or disconnected you say everything works the WFCO is functioning. The problem is somewhere between the power center connection to the battery or the battery itself. I suspect the wiring as you say no electrical with the power center turned off.

When disconnecting the battery allowing the fan to stop EXACTLY where are you breaking the connection? At the power center end or at the battery end or somewhere else.