The black water holding tank is supported on a uniquely styled Styrofoam bed inside of a galvanized box underneath the bathroom floor.† In my Overlander, after water leaked in and destroyed the bathroom floor, it then drained into the galvanized box where, after many years, the galvanic coating finally gave up, and the box all but rusted apart.
In retrospect, this process was probably hastened through galvanic corrosion caused by contact with the aft belly skin.† It is my understanding that, in subsequent years, the Factory stopped the belly skin just short of the box.
In the you learn something new every day department, I did not realize that 20 gauge sheet metal was so reasonably priced - $22 for a 4X10 foot sheet.
Fortunately, I have access to a four foot sheet metal shear & brake.† Unfortunately, the box measured something like 52 inches wide.† So it was constructed in two pieces.† Although the weight is suitably supported from beneath on angle irons, I made the sizes of the two sections two-thirds & one-thirds of the total width just to be on the safe side.
The original box was tack welded at the seams.† I used 1/8 inch steel rivets, 107 of them to be exact.† Why would I remember?.† Well, I only bought one box of 100 rivets to start the task.
In the subtle department, there appeared to originally be a gasket between the box and the floor decking.† Near as I could tell, it was 3/8X1/4 inch open cell foam.† Although one noted Airstream guru claimed there was no gasket there, I decided to use one because it seemed liked a good idea since the black tank is heated with forced air during cold weather use.
Lastly, in the letís save money without trying department, the previous owner had left a pair of bed rails in the Overlander when I got it.† They just happened to be almost the same size as the badly rusted angle irons which were originally holding the box in place.† I love a low or no cost repair!