Flush longer...replace the joker valve...and...
Use more water.Whatever is in the line between the toilet and the top of the loop is gonna run back downhill into the bowl...you aren't pumping enough times to push the bowl contents all the way over the top of the loop. Getting it over the top of the loop will still leave a little in the line run back downhill, but only a cupful or less, not half a bowlful. A joker valve that isn't worn out will reduce the backflow to very slow seepage...one that's worn won't slow it down much if any.If what's coming back is waste, you aren't using enough water to rinse the waste out of the line after you've pushed it over the top of the loop. If you want any runback to be clean water, you need to put a pint or two of water in the bowl after you've flushed the waste over the top of the loop and push that over the top of the loop. Any toilet that's working anywhere near specs CAN push bowl contents over the top of the vented loop in the dry mode.And btw...there's a much easier way to prevent sea water intake odor (sea water makes no difference whatever in the holding tank) than the way you're doing it--which isn't all that good for the pump either. And unless you "exercise" the intake seacock regularly, it's gonna seize and become inoperable.