Gravel would be good to store the trailer on.
Getting weight off the tires generally means jacking the trailer up, and blocking under the frame rails to minimize suspension damage from sitting. For example, the stabilizers are meant to stabilize the trailer, and should not have a lot of weight on them. Over time, the tires will lose air pressure, causing the stabilizers to hold up more and more weight. Eventually, the weight will be so much that either things like the main door won't open and close, or the stabilizers themselves will fail.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.