Call a Keystone dealer that has a body shop and works on Montana's. They may have a piece of J- panel left over from another repair. I just had the J-panel on the rear of mine replaced. It was about 3 foot. Charged me $100. Seemed pricey to me until I priced it at Trekwood. Hope this helps.
2017 Montana 3720RL Legacy
14' Ram 3500 DRW