You've taken the campervan or caravan out of winter storage, tested the water system — and the tap in the kitchenette is leaking or turning stiffly. Often the result of months of standstill: rubber seals dry out, small parts get coated with limescale.
Taps in campervans and caravans are small and compact. During long periods without water flow, the rubber seals dry out and lose their elasticity. As soon as you pressurize the system again, leaks appear in places that used to seal fine.
When storing for winter next time, don't let the tap stand completely dry — a small amount of water in the system keeps rubber seals supple. Consider frost-free storage if possible.
Make sure you're sorted before departure, not on the road.