How do I fix plywood floors with sags sinkholes or craters in them.There are four craters in the floor of a permanent trailer home in a park.The floor is some type of wood actually I have no clue?

The floors in most trailers is chipboard. Sawdust and glue. Drop something like the leg of a bed or table and it cracks it and over time it will sag. In the past, I have used sheets of metal to go over the hole or sag. The metal needs to be enough bigger than the hole so that you can nail or screw it into sound floor. You could fill the dip with floor leveler used to even out floors before linolium is put down. Another option is to go underneath, cut the fabric sheet that covers the bottom of the trailer and add a 2x4 or 2x6 edgewise under the sag. Possibly put a piece of 3/4 inch thick plywood up first and then the 2x under that.