Originally Posted by mrbrzmc2
....... Also, how does one empty these smaller holding tanks into the main holding tank.
You're going to have to explain that one a little more. If you have multiple holding tanks and they are intended to drain or be pumped into the "main" holding tank, you should be able to see the plumbing and pumps (if applicable).
Often, if there are multiple holding tanks on a boat, each has its own deck fitting and overboard pump and thruhull.