Greetings Sandy from Minnesota!
This forum is loaded with so many knowledgeable people, just ask and someone is sure to respond. I am still a newbie and am learning a lot from reading other people's questions and learning about RV'ing. I might assume when you say 1/3 full, it is the monitor panel lights that still show the gray tank 1/3 full. Could be a dirty sensor causing the meter to read like it still has water in the tank. Might be able to spray water back up the drain to clean the sensor (they have special fitting where you can connect a garden hose to back flush the tanks). Good Luck!
Welcome to the forum!
2014 Salem Hemisphere Lite 282RK
TV - 96 E150 Van 5.8L V8
Tekonsha P3 Brake Cont. w/EAZ-Lift WDH