Often such leaks are the result of debris getting trapped between the blade and the seal when you close the gate. The debris may still be jammed in there, compromising the seal's ability to completely shut off flow; or the debris tore the O ring. You may only need to disassemble and clean the gate valve, or replace only the O ring.
Rigs that are stored for long periods allow seals to dry out, so lubricating gate valves and toilet valve before storage is important in that scenario.
After dumping, flush rinse both tanks with extra water from the toilet and sinks (use the built-in rinser if you have it), and as Leah recommends, add a gallon of water and a lubricant like coconut oil so it settles around the gate ports a bit. Move the valves in and out and work the lubricant solution into them before closing.
-Joel