[JRTJH]

The anti-siphon valve is probably located beside the faucet connections under your bathroom sink. Open the vanity and you should find 4 PEX lines. One is hot water and one is cold water. They go to the faucet. The other two go "up behind the sink" and are connected to the anti-siphon valve. You "should be able" to push the PEX lines down into the basement to access the valve. Some owners have reported it installed backwards. Try that, if it doesn't work, then look for "kinks" in the line, either between the outside connection and the valve or between the valve and the black tank. .

NOTE: If you have a "outdoor water supply cabinet", you may have a valve to switch between city water/fresh tank fill/ black tank flush. You may have the valve in the wrong position to flush. just a thought since I'm not familiar with how your trailer is "optioned out".....

Good Luck