Replace the joker valve.. and...
Replacing the joker valve will slow it down--might even stop it completely for a while..but it's not the permanent solution 'cuz a joker valve isn't supposed to prevent slow seepage, only a flood.There are two ways to actually cure the problem:1. a loop--not necessarily vented, just an arch higher than tank contents can climb over--in the head discharge line2. move the inlet fitting in the tank (which, thanks to a little doodad called a Uniseal, isn't that hard to do--and costs almost nothing...see it here: http://www.aussieglobe.com/uniseal2.htm) away from the hull toward the centerline of the boat...so that waste runs away from it when you're heeled to one side, the head discharge hose is running uphill when you're heeled to the other side. If you also spill waste out the vent when heeled, moving the vent fitting too will also solve that problem.IMO, #2 is the best solution.