The datasheet of RM35LM33MW tells that the probe cable length should not exceed 100 meters , in that we need to use in 300-400 meters what could be the suggestions ?
This is correct, RM35LM33MW shall work up to only 100 metre probe length as mentioned in the catalog; max cable length between relay and probe = 100m
In order to have a solution for 500m deep tank you have to opt for RM4LA32* (it is valided for upto 1000m cable length between relay and probe):
Max cable capacitance = 200nF ; the sensitivity range for long cable length has to be selected between 250ohm to 5Kohm.
You should only use is a 2 core cable with common cylindrical sleeving ( max diameter about 0,25 inch ( 6,3mm ), ensuring the voltage drop across the cable and the parasitic capacitance are within the range .