The first thing you need to do is Air the trailer out. the main reason for mold is a damp area and no air to dry things out. Just like when they are collapsed and sitting. Once the trailer is fully dried out There are a few product that help such as Totally Awesome from Dollar General this product has been reported to work very well on moldy canvas.
If you are not around one of these store then some good old home made recipes such as this one. Use the bucket to mix a solution of one part bleach to eight parts warm water. Good old Bleach comes to the rescue once again. Just use a spray bottle to lightly spray the bleach mixture on the affected area. Always taking care to not overspray areas you don’t want bleached. Make sure you air is out for a couple of days after applying this method..