Depending on the brand and style of composite decking material, it can be very heavy if it's the cheaper stuff, or very expensive if you want the lighter stuff.
One caveat to stuff like Trex or Timbertech, etc. is weight, and the fact that it sags if left unsupported. It has to be carried (laid) on 2x material on 16 inch centers, IIRC, it's been a couple years since I've used it.
Colors are very limited, as well.
And again, some of it's HEAVY!!! Do you really want to add all that weight to the top of your vehicle?
I'd rather go with 5/4x6 wood decking, if I could afford it I'd go cedar or redwood (light, weather resistant), it would afford drainage in heavy weather, and will add cooling shade in the summer sun.
There are hollow
extruded plastic composite decking materials, but again you pay a price for the extrusion process.
You CAN stain Trex and some other deck materials, but if you do so, how do you keep it from getting all over the bus?
And NO synthetic deck material, to my knowledge, is considered to be safe as a structural material, in other words, you can only use it for decking and maybe post/railing material. A structural deck frame has to support it.
I recommend contacting the manufacturers and dealers for more info that's up-to-date on recommended applications and color choices.
Synthetic deck materials have been covered in articles by Fine Homebuilding magazine, The Journal of Light Construction, and many others. Google synthethic decking.
You can also check the resources list of This Old House (pbs.org) for their uses of these products, with an index of suppliers.
BTW, speaking of deck design, how about placing flush tie-down rings on/in the deck, to allow for lashing down loads? Just make sure they're rated for lashing.......