My girlfriend recently picked up a used, year-old, custom built(by Wells Cargo) cargo trailer conversion. WC installed insulation, 50A electrical service, internal LEDs, white vinyl paneling, AC/heat, e-track, windows, RV screen/door combo, awning, ceiling vent..
I recently installed hinged on e-track fold-up panels that form a 68 x 80 inch sleeping platform. The platforms are functional at the moment, but not finished. One platform or both can be set up, depending on needs.
There will be cabinets and a counter-top built in the v-nose area forward from the side door. Still drawing those up..
The 'simple' part of this build is that there will be no water systems, toilet, shower, sink, tanks..etc..installed. We're long-time tent campers and are comfortable without the extras inside the camper. There will be 12V & 120V electrics built in as well as solar panels for recharging, ceiling fan, USB recharging, 12V plug for a laptop, 12V fridge/freezer(ARB, Dometic, TruckFridge..etc).
The base trailer & recently built-in bed platforms & vinyl floor are below.

Modeled bed platform build

Built bed platform (still work to do, including covering platforms with some padding and fabric):



Both of the following are mockups that got us camping last weekend..I'll rebuild both with a bit more attention to aesthetics:

