I have a 1988 45' Californian and the fuel selector manifold is indeed under the last step of the stairway leading to the master stateroom. You can see the lines and the back of the valves on the port side of the engine room just above the prop shaft.
My main tank (210gal under the aft stateroom bed) had a bad fuel gauge. Simple float gauge and i found one to replace it with on Amazon for $25. Very simple replacement.
https://www.amazon.com/Bosch-SP0F000...s%2C171&sr=8-2
The side tanks (next to the engines) are around 92 gal and use a capacitance type gauge with a "tube inside a tube." I thought mine were bad because after several weekends of use they still showed almost "full." I unscrewed the top screw on the fuel gauge probe (on the side near top of tank with several wires attached) and fuel shot out. So it really was almost still full! Ran it a couple more weekends and then pulled the fuel sender. There was room to get it out and pull upward. I re-attached the wires and the gauge showed "empty." I had the wife watch the gauge and it went up to almost full gradually as I inserted the probe. So they work! But the company that made them still will make them if you order one. I think they need the measurements of the old one, or if you call them - they may say send the old one and they can copy.
CENTROID PRODUCTS - Electronic Fuel and Water Senders