Jim,
The dual filters I have are close to Bob's set up... with a small difference.
I put a Tee fitting on the intake side with a Hose bib valve on one end of the tee... and a female hose fitting in the center part of the Tee fitting.
This gives me another valve to use when I want to dump the black water... using a short hose connected to the Sani Flush. No need filtering the water going into the Sani Flush System.
I have been told... using a filter body that was clear... could allow algae to grow inside because of sunshine reaching the water. I don't know if it's true or not... one of mine is white plastic... the other is Stainless Steel connected together with a coupling.