I agree with Ed...there was no siphon. It took pressure to send a tankful of waste all the way back to the toilet...and flushing a toilet against a blocked vent pressurizes the system. The toilet was hard to pump because the pressure was creating tremendous back pressure...that's ALWAYS the first clue that the tank vent is blocked. If it was sometimes hard to pump, but sometimes not, it was because some of the pressure was escaping through the toilet between toilet uses.
So there's no doubt in my mind that your tank vent is blocked, pressurizing the system enough that you're lucky it didn't erupt in someone's face while pumping the toilet, or burst the tank. The tank also had to be full to overflowing (I'd bet real money it had been overflowing out the vent) to send that much waste back to the toilet...which is a good thing, because if you'd tried to pump out a pumpout would have pulled a vacuum that would prob'ly have imploded your tank. So you have a mess, but no damage.
The thru-hull may not be blocked, but did you also check the vent line connection to the tank...both that end of the vent line and the vent fitting on the tank? The odds are good that the flood relieved the pressure, but to be on the safe side, open the deck pumpout cap very carefully, with a hose at the ready BEFORE you remove any hoses from the tank. Clear out the fitting and the end of the hose. Then check the thru-hull again...scrape it out with a screwdriver blade or whatever works...if there's a screen in it, knock it out. Better yet, replace the "vent" thru-hull with an open bulkhead thru-hull that will allow you to prevent future vent blockages by sticking a hose nozzle up against it and backflushing the the vent line every time you wash the boat. (I suspect you also need to install a tank level indicator, but one thing at a time.)
After you get the mess cleaned up in the head and the bilge (I recommend using a power washer and a lot of detergent, followed by a very thorough rinse), apply a fine mist of PureAyre (a pump garden spray jug works best) to every surface, nook and cranny...DO NOT RINSE! Just let it dry, with all hatches open...turn fans on if possible, Remove cushions and softgoods as soon as possible to prevent odor from saturating them...put 'em out on deck if the weather is good.
The solution is to loop the hose several inches above the level of the tank or make sure the inlet for the tank is on the top of the tank and not one of the four sides.
That does a nice job of preventing waste that gets into the toilet discharge line when the boat is heeled from getting back to the toilet, but it wouldn't have prevented waste under that much pressure from getting back to the toilet.
