Found this text on a RVIA website:
"Anti-Siphon Trap Vent Device (ASTVD)
The anti-siphon trap vent device (ASTVD) or check vent depicted in Figure 2-60 is a
secondary vent that can only be used in applications where an exterior vent is also provided.
As already stated, the venting system allows air into and out of the drainage system.
The ASTVD uses an atmospheric pressure-controlled valve that will allow air into the system,
but not out of the system. Because it does not allow air out, it must be used in conjunction
with a primary vent, i.e., exterior vent.
The seal of the ASTVD needs to be positioned so it is at least 6 inches (152.4 millimeters)
above the fixtures trap arm. The ASTVD screws into an adapter fitting that is a permanent
part of the drainage system. When installing or replacing an ASTVD, be sure it is only hand
tightened. Overtightening may crack the adapter fitting. ASTVDs that allow gases to escape
into the RV should be replaced."
Link to large PDF file: