Canvas Waterproofing

Ron,
Generally all you need do is give the canvas a good hosing, leave it in the sun to dry then repeat the process a couple of more times - this "shrinks" the canvas especially around the stitching.
As the unit is second hand perhaps the previous owner had already done this.
You could also run a dry, unlit household candle up and down the stitching - "belt & braces".
DB

if i may offer one piece of advice..

don't be too quick to 'waterproof' the fabric, by all means do as suggested above with both the wetting down and seam sealing, but if the fabric is still sound, don't apply any agents to it.

these waterproofing compounds reduce the canvas ability to breathe, if it ain't leaking, leave it!!

I've had a Cub Campomatic and then a Supamatic for years and when the water starts to seep through the seams in heavy rain I put it up at home, get the car wash brush on the hose and give it a good scrub paying particular attention to the seams. This washes the dirt out which is the cause of the majority of leaks. When it dries it will be totally waterproof again. My first campomatic is still going strong with a friend after 25 years on the original canvas.
To maximise the longevity of any canvas, camper or tent never cook in it or near the canvas, don't use flyspray or other sprays and a biggie especially when the kids are doing it is don't wash up or use detergent near canvas, you will never get it waterproof again if you get a spill.

Thanks to you all for the advice. I think this is a terrific resource to be able to get such practical knowhow.

Best Regards

Ron