How old is your trailer?
Had you leaked tested it by spraying it with a garden hose to try leaks
before storing it?
What did you cover it with?
What state do you store it in? (AZ problems vs FL problems vs MI problems might be quite different)
Were all the windows
and roof vent(s) closed tightly or did you leave some cracked open for ventilation?
Do you use a heater or dehumidifier or damp rid?
Do you think amount and location of the water might be condensation or are you sure it is a leak?
If it is a leak, good luck! They can be interesting to track down.