In a utility sink or tub I completely soak in warm water with Thymox added. Thymox is a really decent botanical anti-bacterial. Scrub any stained area with a brush. Put in washer and spin the water out. Rinse several times until rinse water is clear spinning each time. You can do all the rinsing in the washer, but don't agitate. Final spinning several rounds. Dry in the sun, turning every few hours and turning inside out as well. It may take several days in the sun to dry thoroughly. Treat leather(I use Obenauf's Heavy Duty LP) and rewax if you need to.
I think the two things that make this work are the Thymox and spinning in the washer to get all the dirty water out.
Some of my canvas packs are from the 1970s.