Kevmar,
I am going to take a shot in the dark here. I am guessing that the main problems are how fast your holding tank fills and how much freshwater you use for flushing??
I too am in the camp of staying with freshwater flushes. Having had boats with both (salt and fresh), I can say that any stagnant saltwater will smell after sitting (in the line or bowl) for just a few days. You might already be doing this, but if it's yellow let it mellow and only flush when the bowl is getting somewhat full. Add a deodorant to the bowl after each "pee" to combat any urine smell. I use CP marine toilet cleaner mixed with water for this purpose (works well and good for the tank too). Also, any TP used for urination is put into a garbage can with a lid. Flush a 1/2 bowl of freshwater just before bedtime to "clean out the system". In this way, the holding tank lasts much longer as does the freshwater supply.
Just a thought.