I am surprised that your dealer cannot plug into 50 amp and recreate your issues. Here is a link to Fleetwood Technical assistance. Maybe you can get some help there.
https://www.fleetwoodrv.com/technical-assistance
The other thing I would suggest is if you are already not a member, that you join DOAI (Discovery Owners Association). This is for Discovery owners only, and there are many very knowledgeable individuals that monitor the forum and will reply to your post fairly quickly. Here is a link to DOAI
https://www.discoveryowners.com/
Also we took our 2018 Discovery to the Rev repair center in Decatur this summer for warranty work and received great service. I know you are out of warranty, but they would be able to diagnose your problem. Here is a link to there service center.
https://www.fleetwoodrv.com/contact
Hope this helps
Jim Pinkerton
2018 Discovery 40D