I don't own one, so hopefully some of the Marlin owners chime in. For fresh water: run the fawcetts until the tank is empty (I don't think there is a drain). Then put a couple of gallons of anti-freeze in the fresh water holding tank and run again until it comes out each fawcett including cockpit shower and, if you have it, fresh H2O washdown. There is mixed opinions on the hot water side. Some access the heater, and drain it, connect the inlet and outlet lines together to make a loop bypassing the heater and run those fawcetts to get antifreeze throughout the lines. Others just run until empty and add extra anti-freeze so that the water heater fills up with the pink stuff. This is probably the easier way, but you use a ton of extra anti-freeze that isn't necessary if you drain it. Air conditioning also has the water line for cooling, which you can put anti-freeze in, but I don't know how. I don't believe the unit itself requires any attention other than that. Again, I don't own one, so I am not an expert on the A/C. Does the owners manual say anything about it?