On my 2008 production Bighorn the fresh water tank has a piece of 3/8 inch outdoor rated plywood under the fresh water tank. During a major I beam frame fix in 2012 (?) I found that the plywood had sagged too, making a place for a puddle to form and start wood rot. I put a wood sealer on the plywood, and drilled a drain hole for any possible accumulated water at the center lowest point of the sag, before the servicer re-installed it.
I have been told that since my early Bighorn build, the factory no longer puts a plywood base under the fresh water tanks.