Don't know much about your rig but I'll try some suggestions. Published specs agree with your expectations for the rig. There is usually a vent in the roof for each holding tank which are typically about 1-1/2 to 2 inches in diameter, with venting caps or these little wind blown 'fins' on top that encourage fume extraction as the wind blows them around. Should one of these be plugged, and you've been pouring that much water at a time down sinks and toilets, you should be hearing gurgling at the sinks as the air displaced by the inflow tries to escape where ever it can. Additionally, it's very common, particularly in the black tank, for the level sensors to be incorrect. In both tanks the sensors can accumulate crud, fouling the systems to fool the indicator as to the actual level. I have heard many people find that in certain units the sink / shower in the bath is draining into the black tank. I'm thinking that in a long rig like yours, the manufacturer may 'cheat' a bit by dumping all the local plumbing into the same tank, as for instance all the drains in your bath are going into the black tank instead of running separate pipes way back to the tank at the kitchen, which costs a few bucks more in parts and labor during assembly. Why they won't hold the expected amount, and the water is up to your toilet base indicating absolutely full, I have no idea other than the tanks are smaller than specified.
