Re: Fresh water holding tank
Well not specifically your boat, but on most boats the fresh water tank is pumped out using the hand or electric water pump through the sink/head ETC.
If you want to get the old water out and put in fresh because it is getting a little yucky, pump it out and then use some bleach and fresh water to clean it out, rinse/pump out repeat.
A couple of times should remove the slimey stuff and smell if there was any.