Both good tips - thanks.
The doughnut on the top may last indefinitely as I sealed it up before winter arrived with ProflexRV roof sealant. The interior wood doughnut has a crack in it but it really doesn't need to be replaced (it looks very nice).
As far as double wall tubing goes, the pipe that is installed is stainless steel single wall tubing, and I don't know if stainless steel double wall is necessary or even available. When I ran the heater for an hour at full temp there was no issues with the flue surround area overheating or evidence of leaking fumes.