The absolute first thing you need to do, is put this issue in writing to the dealership, and the manufacturer. Some manufacturers only have a 1 year warranty to begin with...if you aren't past that date yet, you're getting close.
The dealership will probably send out a service tech to check the issue - to see if the manufacturer's warranty will cover the problem, if nothing else.
Most double-wide homes have all the plumbing on one half, and quite often it will run the entire length...so think about your home, do you have plumbing on either side of where the floor is soft? A laundry room on one side perhaps, and a bathroom on the other for example? If so, then yes, there is plumbing under there.
Assuming there hasn't been any frozen pipes, any leak that has taken a year to turn up problems is going to be a small one. Possibly, you have some water that has filled a section of the underbelly (the plastic on the bottom of the home). Best way to tell, is to look. If you have vinyl skirting, you can remove a section near the soft spot(s), and see if the underbelly is weighed down in that area. A quick poke with a knife, or screwdriver will let it drain, but I'd wait until you hear from the dealership before you actually do anything other than look for a potential problem.
Speaking of plumbing - some manufacturers use PEX style plumbing - tubing with compressed band fittings - its fast, inexpensive, and easy for them to install - but it costs more for a homeowner to maintain. Plastic compression fittings to match this tubing can cost more than solid brass fittings you could use with CPVC plumbing. Qwest is one brand of fittings for this style of plumbing...your other choice is to invest in the crimping tool itself ($50 to $100), and some copper bands.
There are other possible causes to a soft floor, such as a bad/broken floor joist, sub floor that just misses the floor joist at some point (along its edge), and in a few cases, even raised sub-floor that when you step on it, it is actually going back down to the joist. There's usually a squeaking associated with the last. One last item would be a loose lag bolt or two, allowing the joists themselves to move away from the steel frame...often there is a 'creak' when this happens... and yes, there is a difference in the sound.
Loose lag bolts are easy to fix - a socket wrench is all that's needed, just crawl along the beams and tighten the lag bolts. If any are stripped, you can glue a dowel into the wood, and re-drill the pilot hole, then drive in the lag bolt.
If this is sub-floor that has bowed up off the joist, then I would recommend a Squeak Ender bracket to secure it back in place (I've placed a link below to their website). This is done from underneath, and only takes a few minutes to install. Raised/loose sub-floor is actually a common problem with mobile homes. Quick & easy test to see if this is raised, when you're not sure - put a marble on the floor (or anything round), and see if it rolls away from the soft spot, or into it. Squeak Enders can be found at Lowe's...sometimes in the flooring department, sometimes in the hardware section.
If there is a warped or broken floor joist, another piece of lumber can be glued and screwed to either side of the damaged section. Go at least 2 feet beyond any damage, use construction adhesive, and wood screws, not sheetrock screws. I could go into more detail for various types of damage - if you need, email me once you know what the problem is, with a photo or two.
Once any repairs are made underneath the home, the underbelly needs to be sealed back up. Some service techs try to get away with using a cheap duct tape to do this, and it invariably comes loose. I always used an extremely strong duct tape, thicker than standard, to pull the underbelly back together, and then I would use a contact adhesive, and additional plastic to make a patch that would permanently seal the opening. The tape I used wasn't available to the public, but the Gorilla brand duct tape is very close to it. 3M has the best spray adhesive, and is carried by most hardware stores. A quick tip on using spray contact adhesive... spray both surfaces, and wait a few minutes. When you can touch the glue, and it feels tacky but does not come off onto your finger - then its ready to stick....any sooner, and it will come back down.
Feel free to contact me if you have any questions, and get this in writing right away.
Good Luck