Wood...

I think most vans are constructed using some form of ply wood.sheeting. older vans are similar to old caravans where the frame is made of wood with aluminium cladding outside and ply wood veneer inside.
Modern MH are made using composite sheeting containing a number of different materials. Our MH is 2010 reg but underneath it seems to have a plywood base.

It won't rust but it probably rot instead. The joys of living in a country where it rains 300 days a year

I don't know if I would agree with using under sealer like tar on wood, would it not lock in moisture in the wood and rot it worse???? would you be better with something like creosote or some wood preservative that expels moisture and is less smelly?
I don't know, any views?

Last year I fitted a towbar to the MH. I had to remove the grey water tank at the back to get access to the chassis for the modifications. The pipes from the bathroom enter the tank on the top side through the floor. From looking at the hole in the floor it's construction looks to be made of 2 sheets of ply about 8mm thick with about 25mm of dense foam between them. I am assuming that the manufacturer would have used some sort of moisture repellent compound in the construction of the floor. My fear in using a product now to add futher waterproofing could cause conditions for dry rot. I have read articles from my caravan days where this has happened.
Also from my caravan days I learnt when checking waste pipes for leaks use hot water as the heat expands the cracks making them easier found

If it is rotted wood preservative is of no use, just remove the rotted wood and replace it with a bit of marine ply if you can lay your hands on a bit or perhaps a bit of shuttering ply, make sure all leaks are stopped first though.

much the same problem myself, a small bit in rear corner only noticed when a small piece of thin ply fell down and when i poked about i found dry but rotten timber battens, from what i can see it must have been a leak from the bathroom above it and rain spray from rear wheel.good news killiand just clean as much as you can and replace the ply sheeting and paint it with some black paint testers didn`t look around there for to long. not sure if the living area is of interest to them as long as theres nothing major.
i`m the same as yourself i`ll get at it at some later date!

Cheers!

I know the sink plumbing has leaked (and possibly still leaks - so we have rendered it out of service until further notice!!) but that's great to know that a solution isn't huge! They're the little things that keep you busy eh.. !

Another solution worth considering is to remove the rotten section of the floor, and then patch the underside with aluminum sheeting about 6 inches past each edge using a good bonding agent. Then you can fill the hole with what ever material you like. The reason that I suggest aluminium is that it can be cut with a jigsaw and it does not rust and also it looks like a professional repair.