I was a little leary at first when I was thinking 120 gallons of "stuff" behind the wheels, but then I got to thinking. Basically you will start out with only 40 gallons worth of fresh water. I suppose you could have something in the other tanks, but conservation of mass comes into play here. Basically...what comes out of the fresh tank will end up in one of the other tanks. That means on 40 gallons of water plus whatever other waste you generate. In theory you could fill the fresh tank twice and only then have to empty the other tanks, but that's just theory and not the least bit practical. What it comes down to is you having about 335 lbs of weight from the water (if a gallon weighs 8.36-not sure on that factor). That really isn't that much weight in my humble opinion. Certainly you'll be adding some waste to it, but how much can it weigh?
I'm not sure on what material you might want to use to secure your tanks. My grey tank (no black on the bus) is just a 55 gallon plastic drum. I plan to build a simple drop down bolt on bracket out of some angle iron I have to support it. The solid supports will be capable of holding it, but will really just have the purpose of lateral stability as I plan to "cradle" it in some old seatbelt material. Seat belt material is rated for a 500 lbs static load and some incredible dynamic load so I think it should be fine. It will be chaf-free and more than capable of holding the weight. Should the belts ever fail, the metal bracket will be able to hold it. I know it sounds a little extreme and a little booty fab, but I don't want to lose that tank or have problems with it so I'm just taking every possible precaution. If my idea doesn't work I'll try something else.