I forgot to pass along the REVERSE POLARITY FUSES I have been referring to are located on the PD9280. There is three fuses side by side between the FAN and the ALLEN LOCK DOWN connectors and two of those are the REV POLARITY FUSES. SInce your 12VDC FUSE PANEL is working with Shore Power being connected I dont think this is your problem but look at them real close to make sure all FUSES on the PD9280 are good.
Once again be sure to check the ALLEN WRENCH tightness of the two LUGS marked POS "+". The one going to the battery bank may be the loose one.
Im also showing this larger photo of the PD9260C which has the same setup but maybe you can see the fuses and terminals better
photo from google images
Be sure to take along the correct size ALLEN TEE wrench shown in the PD9200 instruction/install manual