GOOD DIAGRAM !!!!You are over thinking things. There is no need to connect the vent on the "anti-siphon loop" to the vent from the holding tank. The "duck bill" valve in the vent is designed such that neither water nor air can pass the vent, out of the loop. It will only open to admit air into the loop when there is "suction" set up by a siphoning effect. That is why one must "push" effluent through the loop with one's pump rather than attempting to "pull" it through.
The following is a somewhat clearer illustration of your arrangement:
![]()
This arrangement, however, relies on two Y-Valves which can be problematic unless they are exercised routinely. Note, too, that one would always want to maintain the deck pump-out/through-hull selector Y-valve in the deck position against the possibility of a back-flow valve failure/leakage in the holding tank pump (it does happen!).
FWIW...
Some advocate a 'small' vented loop (the vent valve on the vented loop being 'small') on the inlet to the toilet head line; but, that has to be carefully and accurately sized so that the vacuum generated by the heads manual water pump exceeds the in-rush air leakage of the vented loop - this is to prevent flooding though the toilet in case the toilets dry/wet valve is faulty; if faulty the head will flood into the boat especially if the top of the toilet bowl is below the waterline. The inlet line vented loop height is dependent on how far below the static waterline the toilet is mounted and how far the top of the vented loop is above the static waterline when the boat is well heeled over.
