Everyone has tastes and goals. A good starting place is to make that list (especially if there is a woman involved).
Most people want to make use of as much storage space as possible, but I wanted to keep the inside feeling as "open" as possible. I'm not claustrophobic, but I appreciate the open feel over the cramped. It's just me and the dog, so I can get away with a smaller bead and less storage space. I ditched the overhead cabinets along the walls because they just made it feel cramped (and there's just not enough working room above the OEM windows). I stow a small office chair for the desk (opposed to built-in seating). And I have two plastic military footlockers that will be on slides under the bed that provide a ton of storage space. Again, mine is more designed for a single person and and a GSD.
I mocked up the bed, head, shower pan, desk, and kitchen space with modular sections out of 2x2 and camped with various combinations until I found what worked best for me. The bonus was getting to actually camp in the truck to make certain it would hold all the comforts that I wanted. I finally settled on this (rough drawing, not to scale).
Here's the mockup
I'm also paneling the walls to include a 1.5" layer of Sika board insulation. The AAR shelters that the military used on these trucks is one big giant thermal bridge (metal-to-metal-to-metal). The interior walls can get as hot as the exterior skin in direct sun (as well as transferring cold). A layer of non-conducting material (insulation, wood, etc) will break that bridge).
For water I'll have 100gal fresh (two tanks under bed), 10gal gray (external), no black.
*the gray tank is mostly for show, every place I camp allows gray discharge
*no black because of the Laveo toilet (and
nobody likes to deal with black)