Maria,
I know you asked Bob, but here are my comments.
1) my Elkmont is set up like most RV's with a flush toilet feeding into a "black" holding tank, and all other sink and shower drains dumping into a separate "grey water" tank. There is no "by-pass" - in both cases, the waste must pass through the tank.
Theoretically, you can leave the grey tank valve open all the time, but this is not advisable. It is definitely NOT advisable to leave the black tank valve open all the time. If you leave the valve open all the time on the black tank, the liquids will drain off and the solids will collect at the bottom of the tank and dry. Over time, this will form the gross but aptly named "poop pyramid" which continues to grow until the tank completely clogs.
The reason you want to keep both valves closed until the black tank is almost full is as follows. By filling the black tank up before opening, when you do open the valve, the high tank level will create enough pressure / flow to suck all the solids out. Then, once the black tank is empty, you switch the hose to the grey tank and empty it; this allows the sewer hose to be flushed of the black matter.
When I looked at my Elkmont, the only ones at the dealership had the flushing toilet. Based on what I've read here, I don't think I'd want a recirculating toilet.
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