I can't make heads or tails of your description of the trailer (upper/lower half etc) -- maybe post a picture or a link to a picture?
If you have three-across lights, yes, you're right, inner to outer they should go reverse, brake/tail, turn. Note that vehicles and trailers over 80" wide at their widest point have to have red rear clearance lights, which must be on the rear face of the vehicle/trailer as close as possible to the outer edge on each side, but while a taillamp can be used as a clearance lamp because the light distribution requirements are compatible, the taillamp cannot serve as the clearance lamp; those two functions must be separate. This is in the law for safety's sake, the thinking being that you don't want to lose your tail and clearance light if just one lamp fails. And that's why you often see two or three red lights on each side of a trailer (though you still see that on trailers that have their clearance lights up at the top on the same level as the central three identification lights).
If you have three lamps stacked vertically, it's probably best to go turn at the top, brake/tail in the middle, and reverse at the bottom.
There are variations on this theme, though. A few years ago at a technical convention in Vancouver, Canada, I noticed their public buses have a very common lighting provision for three stacked 7" round lamps on each side of the bus (with a smaller 4" round reversing lamp below those), but the way Vancouver specs out the system is opposite of the way I've seen it everywhere else. It's usually, top to bottom, red brake/tail, amber turn, red brake/tail. Vancouver does the opposite: amber turn, red brake/tail, amber turn. When those buses signal a turn or a lane change, it's VERY APPARENT with two big bright amber turn signals flashing. I spent my lunch hour one day on the phone trying to talk to someone who could tell me why. Finally got someone at whatever office it is that specs the vehicles, and he said something like "Yep, we do that on purpose. We changed to doing it this way 25 or 30 years ago to try and reduce lane-conflict crashes when a bus pulls away from the curb into traffic. We saw enough of a reduction in those crashes, without an increase in same-lane rear-enders, that we've stuck with it. Every time we go to order new buses, the maker tells us we're doing it opposite of how everyone else does it. We thank them for pointing it out and tell them to do it anyway."
That seems to go along with some research (UMTRI, NHTSA, etc) suggesting rear turn signals might actually be more important than they might seem to common sense, in terms of crash reduction. I think most people probably think the brake light is by far the most important, but that might not actually be the case...probably because there are other visual cues that a vehicle ahead of you is slowing or stopping (it gets larger in your field of view, and with severe braking it will tend to lower the nose and raise the rear) but there's much less of a visual cue if someone suddenly turns or changes lanes without a signal.
Anyhow...food for thought!
(And yes, there's a market for those brake lights with the built-in reverse lamps because many trailers and big trucks do not come equipped with reversing lamps -- the law doesn't require it -- and do not provide a cutout to put in a dedicated reversing lamp).