I have a 2008 Classic 25fb. You can contact Airstream to see if that is available but I bet if it is the price is going to floor you. It would be cheaper to get a local company to fabricate on for you.
The rear bumper storage is very useful. I carry two 30am cables, two 25' water hoses, coax cable, bags of water and electrical accessories, sewer hose extension and I have room for more stuff.
2008 Classic 25fb "Silver Mistress"
2015 Ram 2500 6.7L Cummins. Crew Cab, 4x4, Silver