Mix a weak concentration of household detergent (1; 20) in lukewarm water. The detergent must not contain solvents.
Apply the detergent and water mix to the fabric using a soft cloth, sponge, soft bristle or soft mop. After cleaning rinse off ALL residue with plenty of fresh clear, cold water.
Always check with manufacture beforehand. Truck wash, car wash, or mineral turps should never be used on any of canvas, clears, vinyl or leathers. As harsh chemicals can take the protective UV coating off, and this will lessen the life of the materials.
With your boat being shut up for weeks or months at a time, you can start to get mildew or mould inside the cabin of your boat. If you can’t leave a hatch window open, due to water getting in when it rains. Leave a couple of moisture absorbers around the cabin, preferable down low. This will help to get some of the moisture that builds up, and make sure you put a plug in the sink, and leave the toilet lid shut. You can pick plugs from any supermarket, or hardware.