Hi Vernon
If it's mildew or mould, which is highly likely up there, the algaecide you get for swimming pools works almost instantly and doesn't affect vinyl fabric, do a spot test if you have canvas. I just dilute about 5:1 with water, gloves are recommended at that strength.

All the best with the next stage of your journey.

Hi Vern,
finally doing the trek from Syd to Whitsundays, currently in Gladstone and loving it. Now after you wash and scrub your canvas, you need to re-waterproof, from bitter experience which every boat owner experiences with bird doo.
Whitworths have a Starbrite canvas sealer, most economical is the 4 litre version. If you spray or brush this stuff on it rejuvenates your canvas, making the colour more uniform and vibrant, but more importantly seals it to make it water proof and more resistant to future stains.
cheers Richard