I see in your profile that you have a 1999 RV.
All Atwood tanks since 2006 now use a 3/4" T&P Relief valve ....your 1999 used 1/2" so you will need a new T&P
New tank comes with retainer rings/gaskets/spacers etc.
Due to old vs new you might need to make some mods to draw pan to fit/align combustion/exhaust pipe.
Check pricing for re-weld vs new tank------especially if leak can be welded without complete disassembly
Replacement tank reference chart1999 vintage......might be better served by NEW water heater vs just tank.
But that would also require new wiring/controls.....120V AC controlled by DC Voltage.