Your problem makes me think you traveled with the trailer in the rain and either the slide was not completely seated against the gaskets or the lower seal has deteriorated and allowed water to get at the side of the non waterproof flooring while you were pulling it. Unfortunately, not an easy fix.
On a different note, if you had it in outdoor storage for four years you are way overdue for some serious roof and trim resealing. Ours sits under a carport when home but we travel enough that it sees a lot of sun and it is inevitable that a yearly inspection turns up some at least questionable sealant.
2020 Keystone Cougar 22RBS, Ram 1500, two Jacks and plenty of time to roam!
The difference between genius and stupidity is that genius has its limits. A.E.
Good Sam Life Member