Like Steve, I'd start with the battery banks - connection integrity, corrosion, chassis ground connection, and internal battery condition. Since you say all is well apparently at the supply post (where I've oft discovered bad outlets), and things work when external power is supplied, I'd consider that perhaps the Echo charger or charger within your inverter, whichever your rig uses, might be out, although I'm not sure you'd be seeing the 13.7v if it was. At 9v. though, the charge rate should be high and the panel should read at first well over 14 volts.
Joel