The OP states: 'The "tunnel" where the burner goes is all rusted out and collapsed.'
Thinking out loud, exploring some alternate ideas:
Could it be the corrosion happened on the burner side of the heat tube rather than the water side?
In that case, the condition of the anode rod wouldn't matter. (Except, during normal use, the anode should have eroded some.)
The air is pretty dry in Mesa Arizona, so one wouldn't think excess moisture would be a contributor as it would be in western Oregon/Washington, for example.
Yet, propane does contain a fair amount of moisture so the air may not have been a contributor.
(Has your propane vendor been padding their profits by doping their propane supply? B^)
Chris, your comments may explain this one - a dry fire with propane would certainly overheat the combustion side of the heat tube.
As enblethen suggested, if I was in your situation, I'd certainly opt for an aluminum Atwood. I've had Atwoods since our first motorhome and never had a speck of issues.