Andy, there are a number of things you can do to test the system. If the thru-hull valves are functional, merely close the valve, disconnect the hose from the top of the fitting, and see if water poured into the ice box comes out the hose - easy test. If not, then either the hose is full of crud, or the fitting at the bottom of the ice box may be plugged with corrosion or just plain crud. Again, easy fix. To check if the thru-hull is clean, while that hose is disconnected, slowly open the thru-hull and see if water comes in. If it does, it's not plugged. If not, then it is plugged.
If the thru-hull is plugged, you may want to wait till the boat comes out of the water before trying to remove the obstruction. Sure, you can do it while the boat's in the water, but it's a wet, messy job.
As for testing the fresh water tank, realistically, and practically, there is no good test. The best advice is to fill the tank with clean, fresh water, and heavily chlorinate it. Then pump the tank dry using your water system. Most of the crud and contaminants will be flushed out of the entire system. Essentially, you will be shock treating the tank, which usually does a great job of cleaning everything in the freshwater system. Be sure to flush the tank several times to remove as much of the heavy chlorine treatment as possible. Standard for treating drinking water is 8 to 16 drops of chlorine per gallon. Shock treatment is much higher.
Now, about that holding tank - you know that if you get caught the fine can be up to $10,000 in some areas of the United States, particularly Florida. There are lots of holding tanks available, and they are configured to fit in some really tight locations. Some are nothing more than a rubber bag, while others are expensive, welded, stainless steel tanks. West Marine has about three sizes of plastic tanks available.
Good Luck,
Gary
