peghall has it - the vent is probably blocked. A real solution would be to switch your septic system entirely to something like (or actually a) Airhead. This is a composting toilet that doesn't even have a black water
tank (so no pump outs ever). I switched on my boat
(not to the Airhead brand - but it's great) and have never looked back. Lots of folks here probably will make comments cause let's face it, bodily excrement jokes are good fun, but a composting system really is amazing in a boat. No stench, no flushing
(so no pumps or hull
penetrations) and no odor
from poop saturated hoses. Two people can use the loo for a couple months before needing to do anything at all. Since AirHead can probably explain it better I'd suggest you g**gle them. Again, I don't own one (mine was built by my grandson on a Norwegian design) or have anything to do with the company but it is the future of marine
toiletriness... what? Oh. OK I'll stop. (seriously, look up composting toilets on g**gle) - AND you can actually install a bidet 'on top'...