[JRTJH]

With the additional information, I'd suspect that either there is a leaking plumbing connection under your sink (probably at the anti-siphon valve) or a broken/defective/leaking valve. They are very easy to replace. If you are still "in warranty", discuss with your dealer and get a replacement valve from him under the warranty. If you're not in warranty, then it's up to you whether to replace the valve with a similar one, go to HD/Lowe's and buy brass parts to replace the bad one or to just install a "straight through coupler" and use a "backflow valve" at the end of your hose. All of those options are "well discussed" in threads on this forum and elsewhere on the internet.

Whatever you decide to do, I'd recommend using a water pressure regulator with the black tank flush system. Too much pressure can damage the system and it's expensive to replace (parts are cheap, man hours are expensive).....

While you're working under the vanity, be sure to tighten up your bathroom sink fixture connections and the drain connection as well. Vibrations from travel can easily loosen them as well as the anti-siphon valve connections.