Toilet pans should not smell as there is a water break between the toilet and the soak away.
You said that the drains smell, do you mean the floor drains in the bathroom, if so these require regular topping up with water to maintain a water break, and should not smell if this is done. Shower traps and sink traps also require regular topping up with water to maintain the water break. Most traps used in Turkey do not have vacuum breaks, depending on the installation the water can be sucked out of the traps when a toilet is flushed or bath emptied. Its a good idea on a daily basis to put water down all unused traps, check to see which ones are dry is a good idea.
As for the soak away, it should never require emptying if it is not abused. It looks after itself, as said previously, using any chemical is a very bad idea as this stops the waste breaking down naturally. Putting toilet paper and other no degradeable products down the toilet is a bad idea also, as this can block the natural draining of the tank, and also form a crust on the top which again stops the tank looking after itself. If other drains are smelling, ie storm water drains, that means that they are connected to the tank, this again is not a good idea as the tank may not be capable of handing the extra demand. swimming pools should not be connected to the tank unless the tank has been designed to cater for it. The foul vent pipe should extend above the roof ridge to allow all vented foul air to disperse.