Please tell me how serious this rust is?!

[TMT]

Seriously, I think this is very scary! Underneath, on the belly, It's a square of rusted metal. Starts about a foot in from the edge. Underneath from about where the original heater is located, across to the steps, back towards the wheels, about 5 ft square. It's RUST, and on the street side, it's actually dropped down about 3 inches. You can see plywood in the photo.

Can someone offer suggestions for repair? How serious?

The original subfloor is in good shape, except right at the front where the window leaked. Otherwise, cannot see damage from top. Of course if you open the cabinet doors under sink, you can see through to the ground.

I'm a little concerned...

[tinbender]

That rusted metal is the frame that holds that piece of plywood that supports the fresh water tank. You can see the tank in the first photo. At a minimum I would say that the frame and plywood would have to be replaced. I would want to take a look at the main frame members that the tank sits between. Could be rust there also.
You're lucky if the subfloor is in good shape. If it was bad , IMO, that would be much more serious than this problem.

[TMT]

Thanks! Sounds repairable. I honestly didn't know what to expect.

[aquinob]

If you have that much rust in the middle of the trailer, the back and sides may be worse. Have you ever pulled down the belly pan to look and see what the condition of frame and floor is? What is your history with this trailer? How long have you had it, whats been done to it so far?

[TMT]

Bob, I haven't had it very long. Just getting started on it. Removed the old flooring and had a soft spot near the front window as we suspected. The window would not close and there were signs of water running down the front. No rot anywhere else we could detect. There was also signs that the front vent leaked as it too did not close tightly. So I suppose water ran down the inside of the shell as there was not water signs on the subfloor under the vent.
Checked all the electrical and gas lines. All that works including the AC blows cold. Even the stabilizers go up and down perfectly! Except for this RUST, and it scares me, the rest of the belly pan is intact with just a small bit of corrosion.
Funny thing, I haven't put any water in the fresh water tank as there is no key to unlock it. May be a good thing there's no key.
Will get someone to take a look this week. I'm in Jacksonville FL and I need to find a good place to take it.
Thanks for your help.