I have the philosophy to avoid anything hanging on the back. If hitch mounted it will be a minimum of 3 feet or more in van length. If so, you will get more bang for your buck just to buy a longer van and get more inside storage volume than you could put outside with a lot more security. I took one trip with a 20 CF cargo shell. It now resides in our daughter's back yard to put ice in and cool beer for parties.
In answer to the question storage in the back is 27 cubic feet stacked vertically and I can get two folding bikes and two inflatable kayaks if I desired. That is more practical and accessible than any storage under a bed that combines use such as sit down dining or sofa. The hiked up beds to get a garage space are nearly as tall as my top bunk and create the either/or situation in having to leave bikes outside when a bed is down.
I'm trying not to have convertible either/or spaces or having to temporarily store inside where we need to walk. We presently use our bathroom to put a collapsible dirty clothes hamper while driving but that goes in the cab behind the turned around passenger seat while parked. That's the extent of moving around things and I envision doing so again with the 144. We turn around our passenger seat almost immediately in our current van when parked.