On a 20 year old MH I would not go overboard with the fix. Does the crack go through the pan? Is it leaking underneath? I would put a header underneath to support that area. If the crack is all the way through I would a fiber glass reinforcement and epoxy or polyester resin. I would put the header under the crack with a slight interference fit to hold up the repair and support the floor after it cures. You can then bond several non skid strips that they make for the bottom of bathtubs and it will cover the crack.