We approached this issue by adding a drain line on the P-trap from our shower which normally would drain into the black water tank in our Hallmark. Then we just run the shower into a 5 gallon jug and dump it appropriately when necessary. That gives us a 14 gallon black water tank and our sink has its own 17 gallon tank we also drain and dump as necessary. Works out pretty well and we dump the black water in regular RV dump stations. Not an original idea as it has appeared from time to time in various posts.