Tony hit it on the head...
Why would you think that odor from flush water coming INTO the head would have anything to do with your holding tank? I've never liked the intake inline devices...they rarely work, when they do it's marginal, and the chemicals in 'em can interact negatively with tank products.Keeping the seacock closed and adding fresh water to the bowl isn't the best solution to intake odor, 'cuz anything poured into the bowl only goes through the bottom of the pump...leaving the rubber parts in the rest of the pump to dry out and fail a LOT faster than they would if they were kept wet. However, there's a very simple and inexpensive cure for sea water intake odor: tee your head intake line into your head sink drain line. Before leaving the boat, and after you've closed all the seacocks (you DO close all the seacocks?), fill the sink with clean fresh water...flush the toilet. Because the intake thru-hull is closed, the toilet will pull the water from the sink, rinsing all the sea water out of the whole system. Or, you COULD keep the seacock closed all the time and run water downw the sink to flush. Just be sure to keep a plug in the sink except when you want to drain it (except when you want to flush using water down the sink)...'cuz otherwise the toilet will pull air through the sink, preventing it from priming.