EF3000iSEB Quite putting out electricity

I just bought a used Yamaha Generator model # EF3000iSEB.  I thought I got a great deal.  I started it up and it runs great.  When I got home I hooked it up to my camper and no juice.  I began to investigate and found that 2 wires running from the battery to the POWER UNIT UR were hooked up backwards.  I checked the fuse on the POWER UNIT and it was blown.  I replaced the fuse on the power unit and still no electricity.  What else could be the problem.  None of the lights on the power unit come on when running.  The only place I am getting any power is the DC out put and I am getting 24 volts per my Fluke meter.  I am a certified electrician and would like to know how to run diagnostics on the boards.

I would greatly appreciat any help.