Your tank is still full
Which means your tank vent is blocked.
A tank vent has two main functions...to to provide an escape for air in the tank displaced by incoming waste, and to provide a source of air to replace tank contents as they're pulled out. When the the vent is blocked, neither one can occur.
So what has happened is, the pumpout could only remove about a gallon before it pulled a vacuum....so when you pumped and rinsed, a gallon at a time is about all you pumped out and then flushed out.
The two most likely places for a vent blockage to occur are the vent thru-hull and the connection to the tank--both that end of the vent line and the vent tank fitting. Clean out both, then pump and rinse and again and I'm pretty sure you're problem will be solved.
To prevent it in the future, backflush the vent every time you pump out and/or wash the boat.
You might also want to check out the link in my signature...it goes to a comprehensive "marine toilets and holding tanks 101" manual that should teach you how to operate and maintain your system to prevent 99% of problems instead of having to figure out how to fix 'em.
