5picker nailed it. There is a selector valve that directs "Waterhose" water to either the fresh water tank or into the system plumbing. This valve is leaking. Turn the water off at the hose connection. Open a faucet to drop pressure. Cycle this valve several times. Sometimes 'o' ring seals can take a set and seep causing what you describe. Try it again. If that didn't work try this; If your valve can be disassembled, remove the spool and examine the o rings carefully. They may be old and dried out. Find a rubber and gasket shop. Take them the spool and they can sell you new o rings. I recommend viton as opposed to butyl. Coat them with a small amount of Vaseline and re assemble.
If the valve does not come apart, new ones are available on Amazon for about $95. At least that's what I recall the price being.
2016 Ford F-350 Lariat CC, 4x4 SRW. B&W turnover with extender. 2015 CC Silverback 29RE, fitted with Reese Goose Box. Amateur Extra Class K5TS, 2016 nights camped 16