There have been some talk about ways to strap/support our waste water tanks, here what I did to my gray tank.
I used a couple old nylon straps, self tapping screws, fender washers. The screws go threw the brace that supports the tank lip. Between the strap and the tank I used some AC foam to soften the straps.
Here is the other side screwed to the other support, this support is lighter weight metal but allows some give as the tank fills. I filled this tank and could hear the nylon stretch with the weight. I will used three straps to each tank.