Unless the lid has been replaced with a paving slab at some point in the past, in which case a metal detector would be as much use as a chocolate teapot.... In any case, follow Andy's advice, if you fall in you wont be getting out again unaided.....
How would a CCTV camera show the exact position of the septic tank, all it would do at best is show the inside of the sewer pipe. ?
A set of rods would at least show some sort of direction the pipe runs and if it comes to a stop this may indicate there is either a blockage or its reached the tank or inspection pit. , the rods can then be measured on ground level as to the approximate position, it would then be worth a try with a metal detector , I had a similar problem like this about a year ago, I used the sons metal detector that he had as a child, worked very successfully. At best it will detect the air pressure vent, venting off the septic tank
If the tank is covered with a concrete slab maybe a garden fork dug into the ground in several places would reveal its location.