I have the same set up as you and my tank has never filled up with inlet seacock open. My indicater on the other hand has a mind of its own and will read whatever it wants to at any giving time.
30-40 gallons is almost 400 pounds with the tank and you said you wanted to mount it on top of another tank. Just research stability issues on boats is all I’m saying
Just curious where do you keep your boat? Or have you had your electronics stolen in the past? I’ve never even thought about locking them up until this point