Ground it to the frame of the trailer. A standard 6' or 10' grounding rod would also be a good idea for a stationary trailer, for both the frame and dish.
Pure, ion free water is an insulator. Contaminants, such as salt etc. cause ions, which increase the conductivity of the water. Hard water, with it's high mineral content is a better conductor than soft water. So, water, by itself, cannot be relied on for a ground connection.I'd love to hear how water grounded becomes ungrounded when it enters a plastic pipe?