Tank: same or different?
So why wouldn't I just replace it with the exact same make and model (Atwood) that lasted for the past 20 years?
Short answer is "yes"... with the caveat that a lot more than the tank goes bad in 20 years. When we did ours, the hoses that connected the tank into the coolant circuit were starting to crack from heat and time and were slightly swelled where they were clamped onto the tank. That's a significant purchase of 5/8" hose right there.
Then, the factory AC wire run to the heater element needed updating to the modern tinned wire (ours was Ancor Boatcable, but not tinned). Also, some of the water supply fittings in that area were due for upgrade -- in general, some of these fittings may not respond well to being wrenched on, disconnected and moved around...
I used that tank replacement project and required opening/draining of the coolant system as a good (and overdue) excuse to replace all the engine coolant hoses and put new coolant in the diesel.
Not your situation, but we had already decided to install a Heatercraft brand fan heater unit in the boat's interior and run that hose loop in series with the water heater tank, so I was combining projects. And then, once you open up the system, you might as well put in a thermostat, etc etc...
I do agree about the attraction of using the same make/model tank, but note that technology may have improved a lot in 20 years and newer designs may be worth your time to epoxy in the old mounting holes and redrill. :nerd:
As to having the aluminum inner tank, quite a bit of gritty oxide poured out with the last of the water when we emptied it, after removal from the boat.
Whatever you do, try to post up some project pics.
As Red Green would say: "We're all pullin' for ya."
Regards,
Loren