Here are pictures of the steamer trunk in 5522, a category 10 "secret porthole" ("SPR") stateroom. The steamer trunks are the same in categories 9, 8, 7, 6, and 5.
Take a look at the pictures below. You'll see that the wall jogs in and out to provide a niche for the steamer trunk. The steamer trunks in back-to-back rooms are actually side-by-side, not back-to-back. Clever, eh? The people who design cruise ships have to make every inch count.
There some staterooms that are against solid bulkhead walls. Those walls are straight. Those staterooms don't have a steamer trunk because there's no indentation in the wall for one.
So, to determine if your category 10 SPR, 9, 8, 7, 6, or 5 stateroom will have a steamer trunk, just look at the deck plan and find your stateroom. Look at the two long walls of your stateroom. If one wall jogs in and out, there will be a steamer trunk. If both long walls are straight lines, there will be no steamer trunk.