[Pathfinder75]

I have a high country 353RL that is similar to your floor plan.
You should be able to see your Black and Gray tanks from your basement after removing rear wall boards. I can see tanks and both cable controled gate valves.
I did the 709-4 kit but I recall they were sending maybe 12" sensors at the time rather than the listed 9" sensors. I did have to cut mine. The sensor instructions tell you to allow clearance from top and bottom. (I believe it was 1 inch from edges. Attached photo (from bottom of tank) of sensor on my fresh water tank. All tanks use 8k wires just the sensors must be coded for location. Instructions make it easy.
I like my Seelevel sensors.