Any scrubbing action will remove the color from the canvas leaving your seats white or grey.
Please read that again before proceeding
You could try getting the seats good and dry in the sunshine, then using a stiff brush, drybrush (more of a flicking action than a scrubbing motion) the mold off as well as you can. Then, make a paste of baking soda and water and liberally paint it over the affected areas. Allow it to stiffen up, then rewet it and hose it off (do not pressure wash, same result as scrubbing. See above).
Mil-spec canvas is treated with fungicides as well as water resistance and flame resistance chemicals. These treatments degrade over time. As the canvas no longer reisist water permeation, the internal foam absorbs the moisture. Putting the vehicle into a mold factory, with no air circulation, like a conex container is a recipe for what you have. Consider what happens in the sealed container on a daily basis with the change in temperature, heating from sunshine, peaks and ebbs in internal temperature, humidity spikes and no air circulation. Ever notice how a tin roof building sweats from the inside of the roof? Even if your MUTT went in dry, it's probably being "rained" on inside the conex.
Retreat with CANVAK, check Cabellas. We used to sell it, but the wholesale pricing for the product (made in Ohio) has gotten it to the point that I'd rather not fool with it anymore. It never was a big money maker; now the retail price borders on ridiculous.
CANVAK is a clear treatment containing parafins and fungicide. It's basically dissolved Wax and zinc napthenate carried in mineral spirits. As it is mineral spirit based, you can add color by introducing a small amount of OD paint to the mix. The key is SMALL AMOUNT. Too much paint will cause the canvas to become stiff and brittle.
Or you can send that MUTT to me and I'll give it new seats and a good home