On composite flooring:
I wouldn't use it if I was replacing a floor in a trailer. It is not a
direct replacement for plywood.
Plywood can be waterproofed. You can get marine grade and you can apply a
product like Thompson's Water-Seal.
The floor is not where I would want to go for weight saving.
The common types of composite flooring do not have high local strength as
it is usually just thin sheet aluminum. If bolts are run through it they
may crush the honeycomb. If not tightened adequately they can work around
in the holes. Reinforcements would need to be bonded and riveted to the
composite floor where the shell frame attaches, and where the composite
is fastened to the underframe.
Al