What Brand?....Suburban or Atwood?
Atwood is aluminum tank.......good weld shop can repair
Suburban is glass-lined steel tank.....forget about welding it.
You need to completely strip all parts off old tank....hardest part is removing the clamp rings that go over the combustion/exhaust tubes in order to remove tank from frame/pan assembly. Then you will need new clamp rings to reinstall tank to frame/pan assembly and swap parts over.....control board, burner assembly, gas valve, gas line, t-stats, AC/DC wiring, electric element, hot/cold nipples.......that's why most times it's just easier to replace with new unit.