about the floating floor... I've typically sneered at them, finding the foot feel to be awful. So I did a glue-down install on my bamboo floor. Well, turns out that Airstreams flex a lot, and also the floor isn't as flat as a typical residential install. Now most of my floor is very solid and perfect, but there are some sections that make a popping noise as after you step on them as the glue releases. Ack!
I guess I'm trying to figure out what the LOA (Least Objectionable Alternative) is in this situation for those that shall follow in the footsteps of this thread!
Anyway, back to the interior walls, which I would call bulkheads, in my nautical parlance. Mine are 1/4" ply, held in place with small L-brackets in unobtrusive places, and lots of Big Stretch caulking. I'm using 1 1/2" aluminum channel, acquired conveniently at Home Depot, to brace up the free edge not held by the side of the trailer. I think that to get soundproofing, you have to either go so heavy with multiple layers, or so thick (taking up precious room), that it makes it a non-starter. When I'm camping, the A/C is running, so that makes a white noise that is the equivalent of sound proofing!
If someone held a gun to my head, demanding an actual answer, I would suggest something like two layers of 1/8" ply with sound board epoxied in-between. A huge bead of Big Stretch around the edges, and Swiss accuracy on the woodwork around the doorway. I'm assuming that you want the soundproofing for sleepers vs. awake-persons?