I'm guessing the chalk absorbs the moisture. when it stops being effective, you pull it out and bake it in the over for an hour or two to dry it out, then put it back in the camper. Rinse, and repeat. It's a nifty idea - I might have to grab a couple of boxes.
To be honest though, we don't have a problem with mould on our camper. It lives outside, all shut up, and the PVC cover gets a bit manky (light bleach solution fixes this), but nothing on the canvas. I mistakenly pumped some water up into the kitchen while the camper was closed for a few weeks over Jan/Feb (wanted the tank filled for Yasi, in case we needed emergency rations), and that made the kitchen and one of the chair bags get a bit of mould onto them, but after wiping down with some antibacterial spray (just the normal kitchen stuff from Coles) they were right as rain, and haven't had any problem since.
We do sit a couple of water jerry cans under the PCV top on top of the canvas though, mainly so that water doesn't pool on the top of the PVC and make a mess, but it also gives some breathing space to the canvas under the top. Maybe it's the heat from the sun during the day that dries out that airspace, but it's always been perfect for us.
Good luck with the Wet and Forget skamme - I hope it works for you and would be interested if the mould comes back quickly, or even at all. Hopefully you've got it sorted!
Thanks!
Matto