When you say lounge windows I am guessing you mean the front windows in the van?
If so, then the fact that they have rotted says that you have a leak, and have had for a considerable time.
If this is the case then the only way of repairing is to strip the front end, find and fix the leak , then replace all the rotten wood, replace the insulation and wallboard and refit the windows.
Whatever you choose to do it will not be cheap and will be very time consuming.
Treated wood available from most timber merchants, use Soudall Fixall as the glue , wallboard from Magnum Motorhomes, new window seals from Seals Direct.