The original schematic for the 75 and later models is in post #1 of this thread
. Note that in the schematic, three tanks are depicted with three resistors--using this resistor ladder allowed the newer tank monitor to use only two wires per tank and still give 4 different level readings.
The earlier 70s system merely connected each tank sensor to a transistor, which switched on its associated bulb. There are four wires to the water tank (one for excitation and three level sensors) and three to the black tank (one for excitation and 2 level sensors).
The only spade lugs (from trailer wiring to the panel, their are others local to the panel) were for the ammeter. All the other connections were via a single large 12-pin connector (4-water tank, 3- black tank, 1-water pump, 2-power and ground, 1-battery, 1-blank). The battery lead was somewhat redundant, but it avoided any voltage drop that was incurred in the power wire.