Ok, under the wire, this is what I have. I built an above-surface bridge, a middle surfaced Living Quarters, and haven't designed yet (but will after the game) a below surface cargo and computer bay area. Afraid some of this might look a bit like luxury liner in nature, but I'm envisioning that the purpose of the ship (as intended by Imperials) being to go on extremely long and far reaching exploratory missions, a bit of luxury would be thrown in for mental sanity. It is usually operated by academy elites, not clone storm troopers, so some lee-way is given.

Though most should be obvious (posts for various officers), the staircase in the center goes up to the Gunwell, and the 2 stair cases that disappear under the center go to the living quarters:

Lockers located in each of the refresher areas identified by the brown boxes. Stairwell going down is entrance into the living quarters from Cargo area/Outside. (no quick access to the bridge). The rest should be easily enough understood... The stairs go up to the bridge area (as per the youtube video), not into the shower areas, those shower areas are underneath the staircases.

Ignore previous post, players pointed out that I forgot propulsion systems which would take up a lot more of that back space. but we worked some ideas and I'll try a recreate this week.

I would say that it looks far to luxurious for a stock Decimator. There's no reason why this ship wouldn't be glitzed up to special standards by some court-born noble with a bribed/purchased Captain rank however. Or it could've been some Moff's private "shuttle" for intimidation.