[Carl n Susan]

I don't know the routing of the toilet line in a 3400. Is it a straight drop to the tank or does it have two 45 degree elbows?

If you have elbows, there could be a blockage in the down pipe they can be cleared with a short piece of hose.

No elbow, then the most likely cause is a pyramid of poo at the end of the down pipe. A flex hose down the toilet can sometimes break it up enough to let the toilet drain. The solution is to use something like Tank Rx along with a tank of water, let it sit for a day or two, then drain. The enzymes should be able to eat away the pyramid and let it flow out.

In the future be sure to use lots of water when you flush and leave the blank tank valve closed until you have a 3/4 or more full tank. The high volume and hydraulic pressure makes for a clean dump of the contents.