Pressure treated Pine flooring is the most commonly used wood flooring material in trailers. The abundant supply of Pine wood in the south coupled with its sturdy nature make for a solid basic flooring choice.
Most utility trailers use pine decking. Sealer will help the decking last longer. Oak does not deteriorate as quickly. I use a good grade of deck sealer on mine every two (2) years.
White oak is typically what is used for trailer floors. Not so much for weather resistance, but for strength. Unless he's hauling some really heavy equipment or driving a forklift on the trailer, might be able to use treated lumber. Cheaper and last longer than oak.
For trailer floors and wagon purposes, marine-grade plywood is recommended due to its resistance to moisture and durability. It's designed to withstand the elements, making it suitable for outdoor use. If budget is a concern, exterior-grade plywood can also be a viable option.
All of the trailers use treated 2x's usually made of pine or else use a steel deck. Tongue and grooved boards are best as they will lay flat and not curl like a regular board does. The treating used on it is for ground contact so it will last as long as your trailer probably.
Any deck sealer will do. Clear or tinted if you choose. If you want something more durable then look to bedliner or epoxies.
You MUST treat the ends and any cuts with 3-4 coats of preservative AND be sure there's a gap between the boards ! ANY 'standing ' water will eventually rot a board. 3rd. clean the trailer top and bottom !,keep deck tilted,in the sun and wind. If it's dirty and under trees, moisture invites mold and bugs and rot...
White oak is typically what is used for trailer floors. Not so much for weather resistance, but for strength. Unless he's hauling some really heavy equipment or driving a forklift on the trailer, might be able to use treated lumber. Cheaper and last longer than oak.
Marine grade plywood is great, but it's not intended to span much distance unsupported. Solid wood boards are much more prone to cracking, but will probably be thicker and able to support more load without deflection assuming you get the right kind of boards.
Just like your deck, fence or other outdoor wood surfaces, your trailer deck requires a sealant to keep out moisture.