This is probably a venting problem, but as my black tank nears full, the toilet will not retain any water after a flush. Water that is supposed to stay in the bowl after flush get's sucked out, leaving the bowl dry.
Empty black tank, everything is fine again.
This is only on this particular toilet. Have standard marine type head in the half bath, never exhibits the symptom.
Any plumbers out there? Anyone else had this issue? Maybe I should regard as a "feature" since I always know when the black tank is nearly full.