Doug,
A thorough rinsing in the spring should eliminate all antifreeze and taste. A 
mixture of chlorine bleach and water will clean and sanitize the tank. 
That being said, I only use the tank water for washing and cleaning. I take 
drinking water aboard in plastic bottles I keep in the ice box or in a larger 
thermos type bottle.
Why worry about cancer forty years from now? You're old enough that you 
shouldn't be alive then anyway.

Rummy  :)