My FW tank was 'secured' with common plumbers straps as well and they left right away. I used 3x3 angle iron (steel) to 'improve' the mounting. Plumbers straps are fine for securing pipe runs in a residence but not for retaining a holding tank (of any kind) in a moving vehicle.
It's the 'lets do it as cheap as possible and send it down the road because we don't have to deal with it' attitude. With empty (and lightweight) tanks, it works. Put liquid in them and plumbers straps fail. Cannot retain the weight, too light.