Bondo works well enough on filling shallow dents and slight imperfections. But, when layed down too thick it will sometimes crack.
You may first try to have the dent removed. One option is to find a paintless dent removal expert in your area. They have tools that can smooth out a dent so it is barely noticable and sometimes completely unnoticable. Another option is to use some of the methods mentioned in previous threads where the tank is pressurized to pop the dent out.