Dmitrit- see if you can pull out any rust "chunks" from inside the tongue. That is what mine had before failure.
Here is my love affair with my trailer:
Tongue gave out on highway due to internal rust- removed completely and replaced with galvanized.
Bunks broken- replaced all bunks, recarpeted and put in new SS bolts (old ones were barely recognizable as bolts)
Sheared off hub (my fault!) Replaced Axle with galvanized axle.
Broke off all lugs to remove lug nuts- replaced rusted frozen drums with disc brakes and new coupler.
Added second galvanized axle, new wheels, new 2 axle fenders, spring hangers, etc.
Stripped, sanded and completely repainted trailer with POR-15 rust preventive paint. Touched up again 2 years later- it is working well.
Removed goal posts. Plan to replace them with PVC (read UNRUSTABLE) posts soon.
Rewired trailer completely with new tail lights and side lights.
Replaced frozen jack with bolt on swivel jack.
Honestly I have lost the tally on the cost and I cringe to think anymore as I may have been able to replace the whole thing with an alumimum trailer. I use it in salt water and wash it off every time I use it. It was used in the NE salt before. I call my trailer Frankenstein as it is almost half galvanized now except for the main frame. I am now convinced that the trailer is actually better than new, even without the disc brakes and second axle.
The moral here is meticulous care of the trailer is required for salt water users to delay the onset of trailer failure. Even with that level of care a painted steel trailer will develop rust .
This is why I think when buying a used Mac (4+ years old), especially in salt water, either take a real good look at the trailer or just factor in buying a new one or spending a lot to keep it alive. (My lovely experience) It has been my single biggest expense by far. The boats are likely to be fine unless they have experienced major damage. I even had a ballast leak that was fixed with $30 epoxy easily. Heck, a few grand more and I could have had a brand new motor....

Oh well. Trailer is a safety issue and I don't plan to have trouble with it again! Taking it 2500 miles tommorow (well, at least starting tommorow) will test it out pretty well.