The question I have about your picture, is are we looking at the main frame rail, or is this an additional piece of metal that was added to support the gray tank? Also, I just noticed that the thread says "Gray water rust repair," but the picture is called "Rust black water tank." Are we looking at a modified trailer with a gray tank, or the original black tank?
Looks to me like it is rusted completely through in at least one place. This is bad, but if it is an additional piece of metal that was added during the gray tank mod, then it may not be very structural. If this is the main frame rail, then worse still is the fact that whoever did this gray water tank install put a big old hole right through the frame rail to run the drain port out to the side. This would be a very bad thing even if it weren't rusty.
No easy fix here. Not only would you need to drop the belly pan so that you can find out where the gray water tank is leaking and causing the rust issue, but you would need to reinforce the rusted area, and probably reinstall the gray water tank mod that the PO put in place.
Then again, it is all a matter of what condition the rest of the trailer is in, and how much they are asking. Just about any 40-50 yr old trailer is going to have rotting floors, rusting frames, mice infestations, and non-working appliances, and dealing twith them is all part of the vintage experience. If the seller is claiming that this trailer is refurbished and has priced it accordingly, you should ask for the documentation that shows what was done (ie., reciepts, pictures, diagrams, blogs, etc.).