Attwood water heater tanks are aluminum coated with some kind of protection over it. Now water from all over the country is different. Some places have loads of iron in it, and that will eat off that layer over time, then cause electrolysis to attack the tank itself. If you use anti-seize compound on the threads of the anode rod it will never seize in the aluminum. But then the plastic plug is cheap, and if you regularly flush out the tank at least once a year, barring other issues it should last for a long time. The key is keeping it clean and carry an extra plastic plug with you.
I was not aware the solenoids are rated that high, nice info to have. I purchased my replacements from Aubrey’s, and tracked down the original manufacturer of them. Here is the link to the company who made them, call them and see what the replacements are, please post information here: http://www.gcvalves.com/index.htm
I did clean up the old ones and kept them as spares, but did not test them after I finished. The filters installed in the coach do not I don't think filter the water prior to it entering the solenoids, so I use an outside filter right of the water supply to stop debres that might be in the water. I started this process when we left home in late October, and I need to change it I think as it's starting to take a long time to fill the tank.