How do you repair soft spots in mobile home floors?

You have two options; option a: Crawl under the mobile home and locate the soft spot(s). Remove the insulation and anything else in the way. Then use framing lumber (such as 2 x 4's or larger) cut to length to run perpendicular (90 degrees) to floor joists. Nail or screw them in place (maybe even use joist hangers) and voila' a new solid floor. option b: This is my preferred method especially if you found the floor to be very rotted out when you crawled under the trailer. From inside the trailer you need to cut the flooring as close to the walls as possible (usually within an inch and a half) then add framing lumber (see above) nailed in such a way that it will support new plywood sheathing. Screw or nail the new sheathing in place with a little bit of construction adhesive on each joist for the first layer that lays directly on top of the joists and voila' you have a new solid floor. ( This example being if you have an entire room such as a laundry room that is bad)