Hiya robins,
You definitely do not need to raise the Bus in order to remove the engine.
Before doing anything else, make sure that the transmission is firmly supported in place. A jackstand or lumber props are well used here. If it moves downward while the engine is out lining up the engine for reassembly can be a bit of a pain.
Undo the bottom bolts on the engine hanger bar (both sides). Simply place your floor jack under the engine and raise the cradle of the jack so that it touches the bottom of the engine. You want the cradle of the jack to meet the bottom of the engine at the oil sump plate or just a little behind the oil sump plate. That helps the engine (with exhaust and hanger bar) balance on the cradle. Once you have the cradle of the jack on the bottom of the engine raise the cradle just enough to take the weight of the engine. Check to make sure the engine seems well balanced. Remove the nuts/bolts holding the engine to the transmission.
Keeping one hand on the engine to help with balance, raise the engine just a bit more. Now pull back on the jack. The engine should move with the jack as you pull back. Move back only a few inches to clear the engine from the transmission. Now let the engine down. Once it is all the way down you can pull it out the back. You will probably have to move the engine about a bit in order to clear the rear apron supports.
Basically (except for swearing and screaming) that's really all there is to it.