Coincidently, I have thinking this same subject, thanks for posting. These are other peoples photos, but saved in my archives.
The tire bin is from Camp-Inn. They will sell it separately.
another storage...
Depending on standard sizes (vary), something like this concrete mud pan, could be used if storage items were not heavy. I am sure other standard plastic bins could be reseached. It's water resistent from the road side, and could be replaced with another if an accident were to happen underneath.
This is Steve Fredericks dropped floor build on his Rondack Lodge. It could easily be covered with a lid for a storage area.
Looking for other ideas as well.