The vented units are designed to release water pressure externally when the tap is turned off. Supposedly, that is to prevent them from freezing up in cold weather. Since my wet bay should never experience freezing conditions, I am going with a non-vented, inline 1/2" IPS backflow preventer and adapter fittings to 3/4' hose thread. It is a little bulkier and a bit more expensive, but won't release water into the wet bay.
Some have not had issues with the vented add-on units releasing water and the speculation is that the Newmar-installed backflow preventer in the tank rinse system is working as it should. I think that is distinctly possible. Since, however, my reason for adding a second backflow preventer is primarily to have a backup in case the Newmar-installed one fails, I prefer not to have one that could vent contaminated water into the wet bay. Overkill, to be sure, but in the case of preventing fresh water contamination, I am all for "overkill.
TJ