If I had my own Sprinter it would be the long one.
I would take out the passenger seat and build a floor to ceiling cabinet. In the bottom of that would be the extra battery. (Freightliner can put in what you need to run all your accessories with, and you know you're safe.)
It would have to sit back just far enough you can see the side mirror without being hindered.
It would also hold clothes, books, papers, food at the minumum. In front of the cabinet would sit a plug in cooler.
I saw a laptop / QC set up on pipe. The base bolted to plywood, a long pipe threaded into that, an elbow on that, and another long piece with a cap into that. Take a piece of plywood, and make a surrounding lip all the way around. Attach this to the pipe on the back side with u-joint clamps and screws. You can Velcro everything and use bungie straps to secure things. (I would have my laptop on it and my phone/phones.)
Right behind the seats I would have a 30" wide box with mattress. It would have a piano hinged pc of plywood that folded up under the mattress, and an extra mattress on top of that. This would be permanent, so there would also be room under it for extra storage.
It would also assure me of a place to sleep if I was under load for a weekend. Then, on LO, I could unfold the plywood, and spread out my bed to be more comfortable. The box unit would then be my bulkhead.
By not having a metal bulkhead, you could put the seat back as far as it goes if you just want to sit up front and be able to look out.
Under it I would store a potty, and camper chair, table, (the kind with pockets in them for drinks. Can use one for ashtray) stove/heater, whatever, and use that stuff in the back.
Along the side to the wheelwells from both directions I would have a piece of plywood, upright to guard them and also store more stuff.
You can store liquids in the step at the sliding door. Oil, washer fluid, water, etc. Tool, straps.
Put in 4 lights in the back at the top, run to your accessory hookup. On LO's put up your chair, table, and sit back and relax.
You can double stack skids, sometimes, so 2 ETracs on the sides is a good idea. 2 on the floor. 4-6 E Trac straps. Loadlocks if you can get them, or substitute by boards and the small locks that fit in the slots.
1 really long tow strap or 2 short ones to go around the bottom of the farthest reaching skid to put the ends on a forklift and drag it out.
Put in one of those roof vents with fan, plus a plug in fan, and you're set.
I ran a plug in fan for 1 week, no power, off my car battery, without a charge. And you can use a household fan of your choice with a power strip. I'm sure you can buy heating units. Just use more juice. So you shouldn't have to idle with a good back up system and yet stay cool or warm.
As you go, put up insulation for winter.
And you can get a front window cover from RV center. Velcro something to the side windows. You feel like you have a lot more room and you can easily get from front to back.
Remember, tire jack is on floor on pass. side. Keep it accessible.
This is work, but it's cheap.
Compare by going back in Archives till you find the Sprinter sleeper unit, or try to find the site that has the picture. It just depends what you want. I wouldn't need a fridge or microwave. But if I had to choose I would get the microwave.
This wouldn't cut your cargo space as much as the unit would. But, having something to lay on when you need to is better than those seats! And, cutting into that space cuts down on the chance you might have too heavy of a load. You can still get 2 full size skids in there, with some to spare.
But after all this, if you'd rather buy a ready made sleeper check out this site:
www.mysprintervan.com/index.php