It will take time. Once you have found the leak. you will have to strip back to sound timber, not all at once, as the front of the van may twist out of shape, decide were you going to start and then strip out a section at a time, then rebuild that section, this will give the van support as you start work on the next section. Take photo's and record measurements, maybe cutting some cardboard templates of fiddly bits in each section, for reference as you rebuild. Plenty of Sikaflex 512 take your time and you will be OK. Seeing that you have rebuilt one corner and it looks good pushes you onto the next.
The worst part of a rebuild is as you strip out, it looks bloody awful, so be prepared.