Toilet on boat doesn't refill bowl

Hi, I have a boat which utilises 2 normal household toilets with the waste going into a vented sewage tank. Both toilets were working fine, but one has recently developed the tendancy to suck everything from the bowl, and not recover the bowl water level. The distance between the toilet and the sewage tank is about 3 feet, and directly below. I don't have any vents in the line between the toilet and the tank, have tried this and doesn't make any difference. Any ideas anyone?

After the flush, the bowl gets refilled by the hose dumping water through the overflow tube into the bowl. That's typically held in place with a clip. If the hose came off or is plugged, the bowl won't fill. It's easy to check - lift the lid and watch after flushing.

What kind of boat uses a regular toilet...a house boat I presume?

It is a houseboats style, but used as a floating bar on a river in New Zealand.

I used to drink lots of Steinlager in my yut.

And it's still a great drop!!!