Additional Floor Support

I suppose Marine Ply is always mentioned because of the glue used but surely exterior plywood could work as well, especially if it was coated with a waterproof paint? Perhaps ply is suggested because for a given width you don't require as much depth? 

David

I would have thought that any wood reinforcement would have to be of a fairly large cross-section in order to be of much benefit. The extra weight of such a reinforcement could reduce the available payload quite considerably. Perhaps extruded aluminium sections would be lighter and do just as good a job. Besides, they wouldn't have to be treated in any way to prevent possible rot.

Ply is a laminate with each layer having the grain running across the layer below. This is the key to its strength. Timber will probably split along the grain and probably a waste of effort. 

Whilst I fully accept that plywood is generally stronger than solid timber, the question has to be "what strength is actually needed to effectively support the caravan floor?" The movement between the floor panels is very small, an estimated 2mm with 80kg standing on the edge of one of the panels. This suggests that excessive strength is not actually needed. I suppose I worded my original query a little wrongly. I should have really asked why is MARINE ply considered the material to use as opposed to standard plywood or solid timber.

As stated in my original post, I have a piece of mahogany, left over from a previous project. It is 12mm thick and of a suitable length and width to do the job. Looking at technical details for mahogany I am even more convinced that it would be suitable for the job. If left untreated mahogany has a life expectancy of 30+ years. It is stronger and more durable than oak, has unusual resistance to moisture and atmospheric changes and is unlikely to twist or warp. The weight, as queried by Lutz, is quite negligible in the scheme of things. It will be less than 1.5kg and probably less than the suggested aluminium section. It will also be positioned over the axle line. 

Seeing as you seem to have made up your mind to use the mahogany (which is used in boat building), Why not Try it and see what happens, you have little to lose.