I have the long van (med roof) and purchased a 60" wide Masterbuilt aluminum hitch carrier for the rear ($90 at Sportsman's Wharehouse). Then I mounted a 36" aluminum underbody truck box in the middle, .....cut.....
I also like that approach. A box off hitch is an excellent way to split the difference between two van sizes. You can use it when needed and leave it at home when using van as daily driver or shorter trips.
For longer trips we often install a modified hitch carrier (has dual receivers) and add a lockable plastic box -- I think it's around 10 cubic feet. We then fill it with tool boxes, water hose, power cord, camping chairs, windshield cover, etc.
Even though it's only 10 cubic feet it frees up a lot of space inside to pack extra clothes and food. And it's also nice to have things you use outdoors outside of van. Lack of outside storage is a downside to most Class Bs in my opinion.
When I see how much an extra 10 cubic feet of space provides, it makes me not want to give up any extra van size. Even so, no matter what van I get, I'll still have a carrier/box combo for occasions when needed.
The biggest drawback for me, other than adding about 2-feet in length, is that it blocks access to rear doors. I can fix that by adding slides to receivers but I haven't gotten around to it. And since the box blocks the rear doors anyway, I've thought of buying a much larger box. When I got mine almost 10 years ago it was the largest plastic box I could find. Now that van is old it's hard to justify any upgrades since we are going to replace it.