Soft floor

Check your roof, windows, etc. for leaks. Get them sealed before you repair the floor.
There's no point working on the floor if you still have a leak.

Consider a pressure test by a RV repair shop, it will find any small leak.

Is it an ultra-light model? It may have a foam core floor that is breaking down

What Make and model is the trailer? Where is the soft spot? Does the floor have carpet or vinyl on it. If vinyl you can tap the floor with the handle end of a screw driver and see if you here a difference in the sound of the soft area versus what doesn't feel soft. If bad will have a soft thud sound verses a sharp tap sound. If sound is different then worth further investigation.

What should you do? Well, that depends. Are you going to keep it, or trade it in? Put your carpet over it, and trade it in?

Sell it the way it is?

Or lift the vinyl up, then the OSB that is likely rotten over twice what you think it it. Replace it back to solid floor joists that have no rot, then cut the vinyl back to a logical spot and put in a patch of flooring boards with nice trim on the sides to cover the edges.

Or do the entire job with new plywood as much as you can, and replace all the flooring.

So..what will you do?

You have not provided enough information. If it is a ultralight with the foam core floor or a conventional trailer?

It doesn't have to be an ultra-light to have a foam core floor. My 2011 Keystone Cougar 24-foot TT was not an ultra-light, but it had a foam core floor that began to break down in high traffic areas when just short of 4 years old. There was absolutely no water intrusion. The glue that was supposed to hold the "sandwich" together broke down.

OP needs to clarify. He said soft floor, leading many to believe a leak has weakened the floor. I read that the complaint is that the floor is just flexing in that location, possibly due to foam core breaking down and all the traffic. The fix isnt easy, so if the only real complaint is the creaking noise, and the noticeable flexing, I'd probably just live with it.

How did you verify there was no water? What you describe sounds a lot like wet plywood.

My PUP floor has felt soft since the day I bought it. I is the materials used.

Our light weight TT floor seems solid.

In the real world, if you are not prepared to replace or repair, you live with it.

For your peace of mind, it would be good to know if the floor is rotting or the high dollar construction is breaking down from normal use.

Good luck

Thanks everyone for your input. The trailer is a Skyline Nomad 22 ft. Of course, they are no longer in business. Anyway, the roof has been resealed last year. I have good confidence in our maintenance company. The trailer is 3600 lbs empty. They didn't skimp on quality. The trailer is in dry warm storage for 5 months of the year and we live in a dry climate the rest of the time. Hopefully water isn't the issue here.