I think saw from a previous post you mentioned you had the 41LKB floor plan which has two toilets, similar to our floor plan.
I’m guessing this toilet is the one located up front and should be a macerator style since it’s not located directly over the blank tank. Ours is a Tecma model and has a similar indicator on the flush button panel using green, yellow and red roughly corresponding to less than 1/2, more than 1/2 and then full. On the Tecma model, when the indicator is red (full) it will prevent you from flushing.
As for the sensors, I’m not sure if Vanleigh somehow tied them into the existing tank sensors for the Spyder panel or used a separate set of sensors dedicated to that toilet. I haven’t gotten around to dropping the underbelly to look at the black tank sensors and determine the location for the SeeLevel sensors yet so can’t confirm if there are two sets of sensors on our black tank. The Tecma manual shows the toilet having two separate sensors being installed on the black tank, you might need to check with the manufacturer to see if that brand is similar.
I haven’t paid too close attention to the Spyder levels compared to the toilet indicator other than when the black is empty or full they seem to match. I think you could be correct about the sensor spacing if that toilet model uses separate sensors. Tecma states the mid tank sensor should be placed halfway up and the full sensor at the top. You may just have to experiment a bit to see how they match up with the Spyder panel levels.
I was able to find this manual for a Sanmarin model 4 in case you have that. It also specifies using two tank sensors.