Done it in the aft head
Brian, on our h450 we have Vac-u-flush heads. Due to the difficulty of getting to the line after the vac-generator and a place to run the discharge line, we installed a valve for the forward head. We installed a new thru-hull close to the existing thru-hull but almost on center line. The valve itself is under the forward bed. A pain to get to since we keep the bed loaded with "stuff". Our intent was to use it in the Bahamas and points south. But in fact we did not use it because we have cats and their litter box was in the forward head and it became a nuisance to move the box to use the head. So the y-valve has never be used (except to try out) and is kept locked in the closed position with a wire tie. Hope this helps.
