Can't figure out drainage on our 99 Sunnybrook!

There should be 2 handles on the left of the drain pipes, and one on the right. From left to right, they are: sewage, shower/vanity sink, and kitchen sink. Perhaps you did not use the kitchen sink enough to see major water drainage? I would take a 5 gallon bucket and pour it in the kitchen sink. See if that provides more flow. Then again, you may have a blocked drainage pipe (highly doubtful).