Floor rot is a result of water leaking in and usually traveling down between the inner and outer shell till it hits the floor. The newer the trailer the less likely to find floor rot, with the exception of some of the FB models had consistent problems around the door area.
Other than going around the perimeter of the floor and checking for softness with something like an ice pick, and then only on carpeted areas, I know of no other way to check for it.
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