when does one have to replace LP tanks?

The permanant mounted tanks are dual wall, that crappy rusted outside you see is not the outer wall of the tank full of LP. I was curious about that and drove my 84 MH to a LP place and they said it's good, don't worry about it unless it's smashed and slaped a new cap on the fill and sent me down the road.
BUT as with all LP parts n stuff you should check it regulerly for leaks and damage.