My 2014 23MBH has a small cylinder on the bottom of the back door that holds it straight out when open. To me the back door is more of an escape hatch than a full time door anyway. It's much smaller than the front and only has 2 steps instead of 3. All we use it for is bathroom breaks while traveling and we use it a little bit during loading.
To answer your real question I doubt it would be very cheap to change but the right amount of cash could surely make it happen