i know this topic has been thru a few times but i thought i'd give 
some recent observations.

I've tried a variety of waterproofing agents ie. silicone sprays,
stuff you throw in the washer, water-based oxysiliane (or something)
sprays, and fancy schmancy sprays in bottles from RV stores.  i have
to say that my favorite for canvas is simply (and cheapest)
Thompsons Water Seal.  this is an old Girl Scout trick.  the Boy 
Scouts don't seem to have a clue on this one (among many other 
things).

paint it on thick with a brush, don't try to spray it.  have it soak
into the fabric well then let it dry.  the can says let it dry for
48 hours before use.  i would easily double that!  after about 24
hours the thing still smells awful you'll think 'what have i done?'
and at 48 hours it will be drier but still tacky.  let it dry a
bunch more and the canvas returns to its original texture but with a
nice ability to bead water on the vertical walls and allow ponding
on the roof sections without drip thru.

the canvas seems to remain breathable.