and congrats on your new vacation palace
I agree about the radio - it should work no matter how you're hooked up.
No slide stabilizers - they are designed to hold their own weight and then some - you.
The fresh water pick-up may not be near the bottom of the tank (so as not to pick up any sediment that may accumulate over time). And, if you're not level, the pick-up may be out of the water but the sensors may be in deeper water.
As for the galley tank, since it is probably located under your kitchen area and then piped to the single grey valve separately, it may take longer to drain because of the distance and the minimal pitch of the pipe to the valve. And, if you're a little higher on the dump side when you do dump, it may take longer. Even though the flow may have slowed to a trickle, it may in fact not be empty, thus the readings you're getting.
Hope this helps
and happy camping.