Nice write up Dave.
I've considered both the Mopeka sensor for FW and also the Garnet, Sealevel sensor system that provides info on all the tanks So, far, I haven't pulled the trigger.
Our FW stock monitor is fairly accurate, but as you mentioned, only down to the 1/3 level. Our GW tank gives us a warning when the shower drain starts makes noise or starts spilling out into the shower.
Our BW tank gives me the most concern. I can't look down the hole to see the water level, as there is a bend in the plumbing.
If, and/or when, you buy that shinny new 855s TC, you will be stopping to dump much more often than in your TT. 