Our first PUP was used and we didn't know the first thing we were looking at so we ended up with a moldy piece of junk. It wasn't as bad as this but it was pretty bad. I called a dealer and they told me what they do. You'll need 2 buckets of water. Put a little bleach in one bucket. Use several old towels. Soak one towel in the bleach water and then soak the canvas with it. Follow it with a towel soaked in plain water. Move on to the next spot/section. Repeat the entire process on all the affected canvas until you get most of the stains off. You will not get all the stains off. But you will kill the mold.
Now, you've destroyed the waterproofing, if there was any left that is. Go to a place that sell tents and buy a couple of cans of waterproof spray for canvas tents and a couple of tubes of seam sealer.
For the floors and non-wood surfaces use 409. For the wood use Murphy's Oil Soap.
2013 Jayco Jayfeather X20 E, 2008 GMC Sierra 1500 Crew Cab Z71 SLE