Firstly where is it wet ??? Water pooling on the canvas top or seeping in the sides ??? I camped in some mighty downpours with just privacy screens and never had any leaks in my 14.44.
I did pull out the mattress and absolutely soaked the canvas at first, let it all dry for a couple of days, run bees wax over all the stitching then sprayed it with waterproofing bought from the saddlery, the stuff you use on Driz A bone coats, just put in a spray bottle and soak the canvas inside and out, let dry, once all done never any leaks........................after a year it will need to be reproofed as the spray stuff will get diluted by the heat/rain/dust etc you can tell as a pillow left against the canvas may cause water to wick thru....just brush the canvas down and respray, easy peasy.
Wetting it all down shrinks and makes the stitching all fit together, the bees wax seals the needle holes and thread, old style canvas waterproofing technique.
Salesmen may say it's all pre-shrunk but even they can't really believe what they say......