[jfaberna]

If you want to replace it, the anti-siphon seems to be a common size, so see your local hardware. For my situation, my sink drain led to a pipe in the wall that went both to the grey tank and the roof vent, so no anti-siphon was needed per "plumbing experts".

So I removed the anti-siphon and replaced it with a end cap to plug it up. I got the advice from other MOCers with the same problem.