FWIW...
I too am wanting to add a Fox Landing to an older (2011) AF 811 and was disappointed to hear the bumper it came with would not support it.
I called the factory, got all the details on the differences on the bumpers and they referred me to my local dealer for purchase. I called and they said it would cost $3800 for the retro-fit onto my rig. That included a new bumper, the Landing & steps and the labor for the install.
$4000 for some steps? Nope.
What I learned (so far):
The primary differences is the reinforcing inside the bumper as posted by the other contributor (sorry, can't recall who, but thanks!) AND the significantly longer and larger "wings" that connect to the TC frame underneath.
Here's a pic of the bumper I bought sitting on a pallet:
https://photos.app.goo.gl/FIVGB7MgKIbbepo63Note that those "wings" are a lot longer and taller than the older bumper style.
UNBELIEVABLY, I found this used bumper off a newer AF on CraigsList that was designed to support the Fox Landing right in my backyard (OK, on the other side of the state actually) for $200.
The give-away is the larger "wings" and the pre-drilled holes in the front of the bumper.
Here's a shot of the current/original bumper on my 2011 AF 811 as it's installed today:
https://photos.app.goo.gl/psKL3byXSPTZro8C3I have yet to install it, but am hoping to do so in the next 30 days. I plan on calling the dealer or factory back to see what they recommend for bolts. REALLY hoping to get away with Grade 8 lags and lock washers.
Will update the thread as work progresses!
Good luck and happy camping!