Hard to tell exactly what the issue is from the pics, but my same 2000 Bounder required similar surgery. And yes, it can be fixed. Just takes some fabrication skills to do so. Beyond a casual DIY'r. You would need to clean the metal (wire wheel / sand blast), reposition the parts that are corroded, cut out the affected metal, make replacement tubes (probably 1" square or so), and MIG / TIG weld them together. Then clean the welds (wire wheel / right angle grinder) and prime / paint.
And certainly fix the leak.
Mine might have been more extensive damage and it took a full day of work. But well worth the effort because it radically change how the RV drove and sounded. A rewarding fix.
The amount of rust in the pics doesn't look too bad. Certainly this doesn't total the rig (unless there are lots of issues).
Hard to give more specifics over a forum. Requires hands on view to assess and repair.
2002 Newmar Mountain Aire Limited 4370 w/ Spartan K2 and Cummins 500hp
ASE Master Certified (a long.....time ago...)