Originally Posted by bikendan
gray tanks have smaller diameter pvc pipes. easy to distinguish between black and gray tanks.
Maybe, if you can see them. Many tanks and their associated plumbing pipes are enclosed.
Mine are and all that is exposed is two 4“ sewage port stubs located side by side. One port has the black and one of the grey tanks plumbed into it somewhere up under the enclosed underbelly. The other is the galley grey tank. You'd never be able to distinguish between my two by just looking at them.