I have held back on this, knowing there are many wiser people than me on this forum.
However, a full 40 gallon tank weighs over 300 lbs, (8.34 lbs per gallon) and if not full, the contents will change centre of gravity in a fraction of a second, making it extremely volatile. (A bit like a 300 lb gorilla)
In seas I have experienced, you don't want this unsecured, and Ronco are wrong! Perhaps they think you are a bay sailor, but yours is a blue water boat. Consider if you are topside, trying to fight your vessel through serious weather, and that baby breaks free. The damage a 300 lb solid object can do in seconds does not bear consideration.
Finding a way to secure the tank is important, as it delays, and possibly nullifies the volatility of the content.