To be perfectly honest, I would use a fibreglass repair kit (or several) as they are easy to work with, albeit a little messy, are unaffected by frost and weather.
All you'd need is to paint a little of the adhesive over an affected area then apply a layer of glass cloth, repaint with adhesive, doing this you build up a few layers until it is as thick and strong as you require. For sealing leaks at seams, you probably wouldn't even need the glass cloth, just hand paint it with a generous layer of the adhesive working it well into the joints. I've used the stuff to repair kayaks in the past and weather wasn't a problem as long as the surface is dry when you apply it then it can do what it likes after it's on, dries to the touch in a matter of a few hours and is fully proofed after 24 hours.
Probably be best to scrub with a very stiff, or wire brush before trying to seal the joints first.
Unfortunately, me being just outside Perth doesn't help you at all, but in an emergency you could erect a small contraption to work under as the work is undertaken a bit at a time ... at least that's what I'd do. It kinda depends on the size of your van as well, if it was a small van it would go under my nephew's carport but a bigger van won't fit. I often do work under flyovers (A9 usually) if the rain is preventing me from getting on with a job.
As a temporary measure, even just getting some gaffer tape along the seams will help reduce the water ingress to an extent.
