Now you guys are starting to make me nervous. Be very cautious about using carbon fiber anything! Carbon fiber is very electrolytically active (unfortunately it doesn't typically show up on a galvanic chart) and will cause the destruction of just about anything metal that comes in pure contact with it. In panel form, the carbon fiber is typically separated from metals by a layer of fiberglass or other fiber material. However, when it comes time to anchor walls, etc. and drilling screws into the the floor panels, the carbon fiber will cause corrosion of the fasteners.
Now for aluminum honeycomb panels. I've used them to make tonneau covers. To be able to do anything with these panels, you must first put an aluminum channel completely around the perimeter and bed it in with autobody epoxy. With the perimeter channel in place you can now mount screws, hinges, etc. to the channel to secure the panel. In the honeycomb panel "field" (as in not along the perimeter) you will have great difficulty mounting anything as there is not enough material to get a screw to bite into and hold. In airplanes, they use backer plates, etc. It won't be easy to secure walls and cabinets in your Airstream. That's the simplistic beauty of plywood floor decking. You can screw things to it.
To evaluate how stiff a plywood is going to be, you want to look at its rated Fiber Bending Stress (Fb). Modulus of elasticity isn't as effective for strength comparisons. The higher the Fb the proportionally stiffer the product. If you want to use a less stiff product because it is more durable, decrease the free unsupported distance between supports to reduce sag under load. You can do this by either spacing the supports closer together or by making the bearing surface at the supports wider (screw a bearing plate to the top of your "C" channels. With a stiffer product (higher Fb) you can space supports farther apart.
Now to get down to business. Unless you are building your trailer as a piece of sculpture and art, fore go the aluminum flooring and use the marine plywood. If you want it to last a very long time you can do one of a few things to the plywood. You can treat it with wood preservative like Olympic Stain Wood Preservative, you can encapsulate each piece by coating it with three coats of West System Epoxy (or similar epoxy) applied with a foam roller, or you can seal it completely so no moisture can penetrate by using boiled linseed oil mixed with mineral spirits to completely saturate the wood until it will absorb no more. Making sure water either cannot get into the wood or cannot do damage if it does get in is the key.