Get a section 4-6" pvc with the screw on ends(up to 8' long). Make you a couple of straps to strap it to the frame and put it just in front of or behind the rear jacks if there is enough room. You should be able to access it from both side if you make it long enough. That way you can put an extra section on the other side. Drill large holes for the water to drain out and let the odor escape. You can paint it black to match the under carriage of the trailer and no one will ever pay any attention to it.
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