Incorrect diagnosis--it's not the joker valve
The only reason replacing the joker valve has helped temporarily at all is because the slit in a brand new joker valve hasn't had anything go through it to stretch it even a milimeter. Once the toilet has been flushed a few times, it no longer seals completely--and isn't supposed to...the ONLY function of a joker valve is to prevent major "spit ups," it's not supposed to block seepage. More importantly, water is rising INSIDE your boat--water that could either overflow the bowl and sink your boat when no one is aboard...or, if it's coming back from the holding tank, make one hell of a mess! Wouldn't you want to know where that water is coming from...how it's getting TO the joker valve that you don't think is working??? The first question is, is the water rising in the bowl clean or dirty (just flush water or waste water)? Does the water rise quickly, or seep in very slowly? If it seeps very slowly, what kind of path does your head discharge hose tank when leaves the toilet...does it run uphill from the toilet? If it rises quickly, where is the waterline on your boat relative to the toilet bowl? Is water rising in the bowl to the waterline? Do you leave your head intake seacock open all the time? What about any overboard discharge thru-hull for the toilet or the tank? When you've answered those questions, I can tell you where the water is coming from and the right way to fix it.As for pouring mineral (or vegetable) oil down the toilet, it's not hurting anything in the toilet, but it does put an oil slick on the surface in your holding tank, which suffocates the contents and contributes to odor...and it's mostly a waste of mineral oil because it all washes out in a few flushes. It takes all of 10 minutes to lubricate your toilet ONCE for the whole season...by simply opening the top of the pump and putting a healthy squirt of SuperLube (thick teflon grease--the same stuff that was in it when the toilet left the factory-that comes in a tube) into it.