505: Fresh Water System

We have a forward tank under the master cabin bed that contains 450 liters and a tank under the settee on the starboard side that contains 180 liters.  Both tanks feed a central water pump.  Here are my questions and issues:

1) Which tank is #1, and which tank is #2 on the sensor panel? Our tank #2 empties first, then tank #1, just not sure which one is which?

2) Once tank #2 is empty and on 0%, tank #1, will get down to around 40% and then the water pump runs for a long time. I am not sure if it ever stops because I turn it off incase it could burn out.  Why can't we access the last 40% of tank #1?  That really cuts into our water availability.  Is our sensor that far off or is this an indication of a problem in the water system?  How do we get access to the entirety of our water capacity?  Does this happen with the fuel tanks too?  I would never run them dry but, how far can you really go down to?  20%?  10%?

3) Does Tank #2 gravity fill Tank #1 or are they totally separate?  I assume separate cause they have two different fills, but I could be wrong.

On my boat, if both tank valves are open, the smaller tank empties first and the valve then needs to be closed to avoid sucking air. The smaller tank will drain into the larger tank if both valves are open. 

Therefore, we open the tanks one at a time, keeping the other tank closed until it's ready to use. 

I replaced a faulty gauge on the forward tank and found it was several inches too short - it showed empty when it was still maybe 25% full. Pretty easy to find a proper length gauge, and they are easy to replace once you line up the screw holes properly. I know some owners have gone to better quality gauges, but the ones supplied by the factory have worked pretty well for me once they were the right sized. 

There is an inspection port in my water tanks that can be unscrewed. I'd have a look inside the big tank and see what's going on with the pickup. But my suspicion is that if you close the valve on the smaller (now empty) tank you will be able to access the rest of the water in the larger tank without sucking air. 

Hope this helps. 

the problem with the reliable magnetic sensors is, that they only come in fixed lengtha ( 50 mm steps ).

You might have a tank with a depth of 480 mm, so your sensor is 450 mm, its hows enpty when you have still 6.5% of the tank volume left. , thats nearly 20 ltr in a 300 lts tank.

The only solution would be to fabricate a adapter flange  which is 20 mm hicker ( if you have enough space above the tank  ).

AFAIK Philippi Germany might make them according to your tank depth, don't know the costs.