My tanks are open underneath except for the galley tank and it has Darco or whatever that stuff is called covering. All my valves are easy to get to. I am poor and have an old camper. I changed the gate valve for the galley tank a couple years ago but to change the black or gray tank would mean unbolting both to gain some space so the two flanges around the valves could be moved apart. I have the valves but have not had the ambition to crawl under the black tank and replace the valve... some of it is because you are under the black tank gate valve when you separate it from the other side and the thought is less than appealing. Perhaps I can talk my BIL into doing it hehehe