Assuming that your vent is open and there's no kink in the fill hose l'd be tempted to run a snake down the fill hose to make sure there isn't something blocking the line.
As for the water heater , I don't know anything about how they do things on your model .. do they actually use a heater tank as opposed to an 'instant on' heater unit ? At any rate , if you have a shore power panel , there ought to be a switch on there somewhere .
Or possibly , if it does have a 'hot water tank' is it possible that it's only set up to run off a heat exchanger from one of the engines ?
Just because something's old doesn't mean you throw it away !