How much water to ad to black tank when starting out.?

The pipe from the tank to the dump valve needs to be filled plus a few gallons more. Some dump valves are right on the tank and some are quite distant, so the answer will not be a certain number of gallons. If you can see the tank bottom, measure the number of gallons or bowl fulls it takes to cover the bottom of a completely empty tank. When using an additive to control the odor, don't add it till the water fills the aforementioned piping so it is in the tank.