If you are considering getting a compost toilet for your RV, . Many people have been faced with this question, because most compost toilets are only for solid waste. This leaves you will the ultimatum of either draining your liquid waste to your gray water tank, or a separate tank for urine.To answer the question, urine is NOT sterile and if it enters your gray water tank it will likely contaminate it. Keep in mind that although your gray water tank already contains nasty bacteria, urine will only make the situation worse. Urine will build up even worse bacteria in your tank, and eventually make the tank smell very bad. Although if your tank is vented properly, odor shouldn’t be much of a problem.