Thetford Tecma macerating toilet
All you RB guys probably have one of these. I love ours, but lately it's not been able to hold water after a flush. The fresh water flows in, but instead of filling the bowl partially (like it does when working correctly) the water immediately (or soon after) drains out. It is not being pumped out by the macerater, but just drains of it's own accord.
This is really strange since there is rubber goose neck pipe (which acts like a trap, I suspect) through which all outbound liquid and solids travel. The top of the goose neck is higher than the water in the bowl. Since water "finds it's own level" it should be pretty much impossible to drain the bowl unless the macerater itself is running.
I've studied a animated drawing on you-tube of how the thing works. There are two "one way" flapper valves in the system, but from what I could tell, they are only there to prevent a back flow from the plumbing back into the stool.
That leaves one other possibility: I've got a vacuum somewhere in the plumbing system that is sucking all the water out of the bowl.