I've had the same problem. My summertime solution was to remove the foam
pad, tape polyethylene sheeting to the canvas just below the pop top to
cover the top bed, plastic on the floor,a nd the van at a slight list
towards the open sliding door. I then sprayed bleach up onto the pop top,
where the stains were. Let sit for 30 min then hosed the top out with
water. The plastic sheeting directed most of the water out, but not all,
so I left van to air in the blistering :) Victoria sun. I even used the
bleach on the "canvas" in spots with no apparent weakening of the
material. Hosing out the top had the added benefit of removing the loose
flocking that previously rained down when the top was touched.

Alistair