Water evaporates, so it's a natural occurence for traps to dry out over time. RV antifreeze is about the only thing you can put in that will last a long time. Do not put regular car anti freeze or petroleum product is a drain ever. If possible, you could occasionallly "recharge" the traps with water, but if you are in a cold climate and the house is unoccupied and unheated in the winter, then the RV anti freeze is best, but then you should also drain the pipes and toilet tanks for the entire house.