If the tank is supported by a method that does not allow it to move, then cushioning would not be required and the tank should absolutely not be moving.
Here you see an area off my holding tank
that appears not to be supported, but both above and below this view of my tank level monitor
are the tie down straps that I referred to. There is an advantage to having some surface of the tank available for monitoring....
I use the SCAD Solo tank monitor
and it has been working well for me with no intrusive device in the tank that can be "messy".
I definitely recognize that Gord May's advice suggesting the tank be supported on all surfaces is wise, but, like all choices aboard, there are compromises to consider.