Well it has been talked about many many times here!! Mostly metal tanks, but I am sure it is all the same.
The Hard part is getting all the foam cut away so you can get the tank out!
After that you are on easy street!
Thank god I no have that problem with my boat!! :)
Here is were it was talked about pretty good at one point and some good info!!
http://www.wellcraftv20.com/cgi-bin/...3553;start=4#4