Your best bet is to find a local fabrication shop and have the J Panel duplicated, and then paint-to-match. Seeing as how your dealer is unwilling to work with you, you could find another dealer. Here's the caveat on that though. If the panel is shipped by truck freight, the cost of the freight will be 2-3 times the cost of the panel itself. That's just the nature of the beast. My dealer has had skirts and panels shipped for customers inside of inbound RV's. Problem here is that you have to have an inbound. Sometimes only part of the order gets put into the inbound. My dealer no longer does this. He uses a local shop to duplicate the panels and skirts, then has them painted to match. Expensive, but still a lot cheaper than the other route from the factory.
'07 K3500 Silverado LT Crew Duramax (LBZ)
2016 Salem 27RKSS
1984 CHEV SCOTTSDALE K20 2GCGK24J0E1XXXXXX (Chevrolet Legends-Class of 2019)
"...exhaust fluid? We don't need no stinkin' exhaust fluid"