I had a trainer once who would only use dish soap to bathe horses. I was so afraid it was going to dry their skin out but I really don't think it did. And it lasted forever.
Another trainer showed me to just use a tiny drop of whatever soap in a bucket of water and bath them out of that instead of putting the soap directly on the horse (much like you do with you dishes). Then you dip the tail into the bucket to wash it. This really saves on the amount of soap you use.