Ronco or Plastic mart have hundreds of different tanks, that can have fittings spun. I would stay away from the ABS tank, it is meant to be in an enclosed area, above the floor.
My trailers now have 41 gallons of water on board, located just above the axle, which feeds on demand to a standard RV water heater with automatic pilot light.
You do not want the tanks hanging below the axle or where it can catch a rock and rip the whole thing off. I would try to keep it within the frame rails and even with the lowest part of the frame, protected on all sides.