i regularly have to rod my sewage inspection chamber,as it has two 90 degree bends[they had no choice before there were rock breakers],just buy a set of rods chimney rods with an extra fitting and do it yourself.
if a septic tank is working properly it sahould not be necessary to empty it
.what you need to do is make sure your bath water etc goes in to a seperate soakaway[still i expect you know all this]