Many caravan supplies stores have waterproofing products like Bradproof, usually applied with a mop or roller.
see:
http://caravansplus.com.au/catalog/product_info.php?products_id=33Although I would try a small patch on the turned under
hem on the inside just to check it doesn't discolour the pattern.
Also, do you have the roof rack? Check that the holes through the canvas are sealed, if you don't have the rack, check that there aren't holes for one, you may want to seal them with either a patch over the top or a screw, washer and flat neoprene seal.
On the sides, I usually fit a dowell or length of coverstrip inside the bottom
hem to reduce flapping and pull the canvas tight against the capping.
Hope this helps.
Diana