its under the caravan its more important that it can dry out between episodes of getting wet than be waterproof
Ah, interesting. It's protected by the body work so won't be continually exposed to moisture.
Does the bit of timber its replacing look like pine, or is it a more resilient hardwood?
It looks more like a chocolate cookie than anything else
I hope these pics make sense. Here's how I think it looks at the moment. Brown = wood, black = metal, blue = insulation, grey spotty = rotted wood, red = the problem member

van problem by molgrips, on Flickr
This is what I propose to do. A somewhat larger red bit so that when I replace interior ply afterwards it'll have something to sit on/be bolted to.

van fix by molgrips, on Flickr
I haven't drawn the screws on, but the trim strip is screwed through the red bit into nothing much at all currently, which is why I have a problem - that corner of the van is unsupported. I need to add a new red bit so I can put some long screws through it into the solid piece behind. Access to be gained by removing lower bit of trim.