Until the wet and rotting wood completely dissolves and is eaten by insects, there will not be a hollow sound. Tapping with a hammer doesnt mean anything
If it's wet, time to repair
I buy a tarp every few months to recover the Fiat. I have a canvas tarp covered by a vinyl / poly tarp. The vinyl / poly lasts a max of 5 months. They do not last in sunlight and wind
Covering a boat, I would go with canvas