I've had good luck with mixing up some Oxyclean in a spray bottle, dampening a clean towel, and scrubbing. Might have to hit tough spots with direct spray to let it sit for a minute. Works on 99% of the junk I pick up camping under a thick canopy of trees.
Keeping the canvas clean was my main reason for getting PUGS. Since I've put them on, had zero issues.
2104 Flagstaff HW27KS
2000 Ford Excursion