It is frustrating to experience what you are going through, especially when help seems beyond reach. So let me try from a distance -:
Troubleshooting electrical problems requires systematic testing of the system, with test tools - don't rely on the EMS, or any other component - Until you have proven they are functioning as designed.
My first guess is you have grounding problem, next would be something in the EMS system, then the Xantrex. First thing - find every grounding connection - take each apart (or verify the connection is sound) and retighten to the correct torque. Does Oliver have a schematic available?
Were it me - I would verify the system voltage - disconnect the batteries and solar - check voltage at the connection where it enters the system - put it under a load - without a load you may not discover the problem. Then work your way step by step into the system. I understand this is probably outside your expertise. I will repeat - the voltage will probably read fine - until put under load. Unless it is in the componentry - but if it reads fine going in, but doesn't come out fine, you may have found the problem, replace and test - you could have multiple issues - but I doubt it.
If the RV service centers are backed up until ----- find a tech on break, at a bar - wherever, explain the issue, discover their expertise on electrical, if none, ask them for names/contacts and offer him/her double triple normal wages.
Good luck, but with the current demand among skilled labor - finding anyone is tough - and most business are so busy they just don't have the time/concern to deal with customers as they once did.
My money is on the simple solution - a bad connection somewhere.
RB - former Oliver owner,
I wish you well.