Welcome to the Bunnings Workshop community @Mark0303. It's great to have you join us, and many thanks for your question about waterproofing a caravan floor.
There are likely several different products that you could use to provide a waterproof coating on the timber. I'd recommend something like Gripset Betta Waterproofing Membrane, as you can dilute it to paint surfaces or use it straight from the container to create a thick membrane. It's also water-based, so it doesn't have any terrible fumes and is super easy to clean up. I'd recommend giving the bottom of the sheets and the edges a coat of diluted product before installation to ensure you get all the nooks and crannies. Once in position, I suggest going over the bottom of the sheets again from underneath the caravan with a nice thick coat straight from the tub. This thick coating will be completely waterproof and durable against wear and abrasion. This product can also be applied to steel to prevent rusting, so you might like to consider its usage over the entire undercarriage.
Remember you're PPE, including safety glasses and gloves.