I'm glad you got it fixed to your satisfaction, but this kind of run around from any other industry would be totally unacceptable.
I think I would have got it fabricated locally. I recently helped some friends out who bought a 1973 Airstream. The floor had rotted, and the chassis had so much rust it was scary to walk inside let alone pull it.
A trip to a custom trailer builder got a new hand built steel chassis using the good axles etc. and what was left of the original as a template for under $2k. I know it's apples to oranges but these quality small fabricating shops are still out there.
I'm a great believer in keeping small business alive, especially when the big ones like in this example 'Keystone' apparently don't give a hoot once they've got our cash.
Now y'all have a seriously Merry Christmas and a Happy, Safe and Prosperous New Year.