I've noticed a problem where when my black tank gets above 2/3 full, the toilet burps when trying to flush. More recently, the back grey tank was very, very full and when the black tank was full to the point of burping, flushing the toilet also burped (seemingly grey) water up through the shower drain. From reading other forum posts, it seems like a root cause may be either the black tank vent or black tank drain being a bit too long, but the shower connection got me wondering about which vents are shared.

Does anyone know if this is the orientation of the vents? There are two on the roof, one forwards towards the sink and one backwards towards where the shower pan and toilet are.
Click image for larger version. 

Name:	tankandvent.gif 
Views:	3 
Size:	26.3 KB 
ID:	33392

It seems like the solution here is probably shortening the pipe(s) into the black tank, but given that it only happens when the black tank is over 2/3 full I'll probably just live with less capacity. Doesn't seem worth trying to pull the pipes and fix it.