Daryl's on the right track...
It does indeed sound like your tank vent line is blocked, pressurizing the tank and forcing waste back toward the toilet. If so, the tank will still be full even though you've just been to the pumpout (and I'll bet you haven't actually checked the level in the tank, just assumed it was empty when the pumpout stopped pulling anything out)...a blocked vent causes the pumpout to pull a vacuum, preventing it from pulling anything out after any existing air in the tank has been exhausted. You're lucky that the pumpout wasn't a particularly strong one...'cuz a strong pumpout pulling against blocked vent can crack a tank. The most common places for blockages in the holding tank vent are the thru-hull and the vent hose connection at the tank. You'll prob'ly need a screwdriver to scrape out the thru-hull. Don't worry about damaging any screen in it...screens cause more problems than they solve, so you're better off without it. If you don't find a blockage in the thru-hull, remove the other end of the vent line from the tank. If you've overfilled your tank, waste build up can clog that up. Before removing any hoses, CAREFULLY loosen the deck pumpout fitting to relieve any pressure in the tank. And have a hose standing by to clean up the mess. If it's not a blocked tank vent, we need to trouble-shoot further. Meanwhile, Do NOT use the toilet again or attempt to pump out until you've found and corrected the problem that's causing your backup.
