Tapping with a hammer (gently) will make a distinct sound on rotted metal. All metal trailer frames and axles get surface rust unless kept painted. The upper picture the frame looks better than many.
In case you did not know a fish plate is a plate welded over the crack weld repair to reinforce the repair. Almost like a splint. Done right will be stronger than original frame.
Couple of image links:
This image the fish plate has holes which provide additional weld connections.
That bend in toward the center that the side rails make is a place that would be prone to break. Not the first picture like yours that have appeared.
Whole lot less work to do as suggested cut small square of fiberglass out above frame, weld and fish plate. Followed by fiberglassing the cut out piece back in.
Only reason one generally needs to replace the axle
is damaged spindle, bent or the more typical reason the torsion rubber is shot from age.
So count me as a third vote for that approach.
I would start by getting in touch with a portable welding service to come out and give an estimate for fixing the frame if you cut out the fiberglass to let them get to the crack.
Figure if a couple of those welding services refuse due to safety concerns after examining the frame then you might need a new frame. Otherwise if they will do the work it's because they think it can be fixed.
Heck Norm and Ginny had a crack pretty much as bad as that under their trailer and it was fixed right in the campground and down the road they went. No frame replacement in their future as far as I know. Done right better than new.
People on the forum will help guide you through the fiberglass cut out and repair. Answer questions and mentor if you are doing something new.