I had the same problem on my 36 since new but of late much worse, so I was also thinking of an electric replacement loo or macerator. My friend suggested that before spending a large sum money why not try some porta poti fluid as this should break down the content of the tank. So far this seems to have worked, at least for the last two weekends and the tank smells a lot better. We are away next weekend for the week so should be a good test of how effective this is. I will keep you posted.
Keith & Carole, Wave Dancer