Holding tank insulation
I don't know about your trailer, but in mine, the bottom of the tank is flush with the bottom of the frame. I'm afraid that most insulations would be subject to attack by the air flowing under the trailer. One possibility is some stuff I found at ACE Hardware, about 1/4" thick, consisting of a foil like substance on each side of stuff like bubble wrap. Another would be that space blanket stuff.
Coming from a different direction, perhaps styrofoam or equivalent, with a waterproof, puncture resistant covering could be attached to the frame surrounding the tank rather than to the tank itself. Pay special attention to the leading edge, to prevent wind from getting under it. I might put a 'spoiler' in front of this.
In any case, a small amount of insulation with a heating element attached to the tank (with the insulation covering tank and heater) might be more effective then a big wad of insulation.
