My gambit has been to use sideways Knaak boxes -- mounted directly to my service body.
Because the expense for a Sprinter -- or anything like it -- is so high -- I favor using a (covered) trailer, instead.
It's only brought into play during a serious build.
The extremely low height available with a trailer is a real step saver.
Wander over to Ask This Old House -- see YouTube -- or This Old House -- to see what Tommy Silva has rigged up. His trick trailer has a collapseable suspension that puts the floor all the way down to the ground.
You will find that climibing in and out of a Sprinter is brutal on your knees during a build. The other advantage is that you can take off for mid-day materials -- while leaving your crew enough (protected) supplies to keep on building.
In many areas, leaving materials out -- even during the work day -- is an invitation for theft.
As for the trailer: have it rigged with a natural skylight for internal illumination (like a UPS van) -- and PV panels, too. These make it possible to top up your battery packs in a secure environment -- and impossible to forget at the end of the day.
For service calls, you fall back on Plan A: just the service truck.
BTW, ladder racks are staggering fuel pigs; figure on a 20% bump in fuel expense when hauling them around. Ladders toted inside a trailer travel pretty much free.
PV arrays provide enough energy to drive LED signage and spot interior illumination... and a security system!
Hence, you're showcasing an item that you'd love to sell and install.
