Yes you can stop it, but you have to weigh the cost and time against the value of the van.
You need to find out where it is getting in, then tackle that problem first, removal of rails, cleaning up and resealing, or maybe replacing panels if they are the problem.
Then the inside.
Removal of all affected wallboards, removal of rotten timber and replacing,drying everything out, sourcing new wallboards and refitting everything.
It is very labour intensive and very time consuming which is where it gets expensive if done by anyone but yourself.
You need to consider if your skills can cope with what you find, as once you start you will find the extent of the problem is usually much more widespread than first appears.
People have done it and done it well, but it needs to be a labour of love.