Ask if the wood is original or replaced, and if it was pressure-treated lumber. The rubber coating can lose its adhesion, and any gap allows moisture to collect on the aluminum floor. Look closely to see if it has started to separate from the metal. If the coating peels loose, that tells you that urine can seep in. A white, powdery substance on the aluminum indicates corrosion.
Corrosion also can cause depressions in the floor, or tiny pinholes.
Urine damages any floor. The horse breathes, produces moisture, and urinates, so the floor has to breathe or allow urine to drain through the floor or out the back. Crank up the front of the trailer and let it drain. Extras Of The Trailer Tally a list of the extras. Start while you're still inside, then recheck the exterior.