Gary3 wrote:
Well it can,t suck anymore its plugged for good.
Actually now that it's plugged it definitely can suck toilet water back into your fresh water system.
Yes it would have to be an unlikely event of the bowl full of water so the fill hose in the bowl is covered. Then at the same time the fresh system has to lose pressure so that water starts flowing back toward the tank. If that happens you do not have a vacuum breaker to prevent siphoning water from the toilet bowl back into your fresh system.
Yes unlikely, but safety things are there for a reason. The manufacturer doesn't just put anti-siphon valves on the RV to pad their wallets.