Tanks

That's a good question. Considering that this would go into your gray tank, I would suggest you use it sparingly. I think this would be no different than any other body product, lotions, etc, that would wash off and stay in the tank as residue. I don't take items like this camping so it's not an issue, but I'm sure that years of soap, shampoo, lotion, etc., residue can build up. I occasionally use a gray tank cleaner in the kitchen gray tank just for the build up from various food items (the product is called Elemonate Gray Water), so the same could be used for the bathroom sink and shower/tub drains too, just to address the issues. As far as what is safe, it's biodegradable it would be safe, but more importantly if you plan to do a safe gray water dump when boondocking, such as in your fire pit to help put out the fire. But, even biodegradable will leave residue.