As previously mentioned, you should start from underneath and make sure the frame and supports are in good shape. Next, why is the floor soft? Has it deteriorated or is it soft from years of flexing under weight
and from travel. Rotted materials should be replaced. Otherwise you may be able to overlay it with new plywood. A minimum thickness would be 1/2 inch but 5/8 would be stronger.
Use a large sheet of cardboard and masking tape to make a template of your floor area to be covered. The template can be made from several pieces and taped together. When finished, the template should fit exactly, otherwise adjust until it does.
Use the template to mark your plywood. If you are using more than one piece of plywood, avoid seams in high traffic areas when possible. Glue/screw plywood in place, paying particular attention to seams.
Laminate flooring is easy to use and very durable but it is also heavy, cold to walk on and not suitable for use on floors that aren't solid. If not properly supported the laminate may flex under pressure which will weaken joints and over time cause them to come apart/leak. Some moisture resistant designs are available but any standing water can be a problem as only the factory edges are water resistant. Any boards trimmed for installation will have unfinished edges which may soak water.
A more viable (and easier installation) option would be to use your template to cut a single piece of linoleum. Apply adhesive to your new plywood floor and lay the linoleum. No seams, completely waterproof, easy to clean and very durable. Don't forget to seal the perimeter.