What Frederick says. I have done mine, have a look here.
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I found the thin aluminum sheeting underneath had deteriorated due to galvanic corrosion where the frame and the sheeting met. I cut out the aluminum sheeting with a uitility knife, and let the floor bottom dry out well.
Once I had removed thing inside, I found the floor a little springy. I replaced areas of dry rot, and replaced the old carriage bolts that attached the floor to the frame with elevator bolts. Then I fiberglassed the whole floor and laid lino overtop. Underneath, I used store bought undercoating, initially from a gallon pail, and final coat was the spray stuff. An area of particular concern was the edge of the plywood floor visible from the inside the wheelwells. A lot of water seemed to have wicked in that way.