Hi,
Just wanted to clarify a common missconception: Canvas is the most waterproof it well ever be the day it is born, IE - your new camper.
The 'weathering' or 'waterproofing' referred to is actually to seal up the holes created by the needle when sewing. When you stitch two sections of material together, the needle passes through the material and feeds or hooks a piece of cotton, to bind the two parts. In this process the original waterproofing quality is somewhat jeopardised. But don't panic, by wetting the seam then allowing it to dry (around 3 times is plenty) the cotton will expand slightly and the hole will shrink to a degree. This is usually enough to create the original seal and is 'weathering'.
If it continues to leak, force candle wax (paraffin) into the seam - to seal the holes. Don't be concerned by this process either, the product that makes a cotton based material waterproof, is actually wax.
I hope this clears things up a little.