Seems to be a clue, that you plugged in "the smaller electrical black connector on the left side of the tank" and then lost all power...

Could you show a pic of that connector or tell us what it is for....???

 

Will do!

 

With that 13.5 on battery, sounds like you just came off the maintenance charger or your meter is junk.

12.7 is the normal. 12.3 is fully discharge . That 12 your getting tells me the plugs you removed taking tank off must have pushed pins out and they really aren't connected. Just berly touching or bent over.