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

I put in about three gallons of water and then ad my solution. I am in this park for a month. My neighbor, a single woman, is the one that is plugged. I advised her that after the plug is over to ad 3 gallons of water. She has been doing this for a long time and should have figured it out.I need to check with her and find out if she is clear. I'm convinced she plugged her line due to lack of water.