Thats how it's done here, trouble is if you have very poor draining soil, you end up with a full tank in the wet season,(this happens at our resort) and your loo and sink willnot empty.
I would invest in a plastic tank and if they can't follow the instructions, installl the sucker myself.
Our(inlaws) resort has 30 bungalows..so multiply your troubles by 30 and look into my eyes.!!.