I would be checking the pressure relief valve and all the rest of the tank fittings before buying a replacement. Atwood heaters, AFAIK, are all aluminum. There is very little chance that it has rusted a hole through the tank bottom like with a glass lined steel tank.
Since the heater is encased with foam insulation, a small undetected leak at a fitting can allow water to travel down the side of the tank and exit underneath.
You may very well have a broken tank but I would check the fittings and especially, the relief valve, before replacing.
