Water coming out of overflow drain under van when I turn on the taps.

This is our first weekend in the camper. I was prepping it this morning, so I filled the water tank and tried the taps. I am getting hot water only out of the kitchen and bathroom tap, and no cold water to the toilet, so I am presuming there is an air lock. I can hear the pump turning on every time.
But when I turn on either tap, I get a slow flow of water, but a lot of water coming out of the overflow underneath the van.

I have closed the drain taps down by the boiler.

Any ideas?

Might look like this.
https://www.reimo.com/r40/vc_r…/72108.jpg

That what to it has be,follow the over flow pipe and you will find the valve.

I tried the frost valve, and no joy. Actually, I have now filled the tank completely, and there is a constant trickle out of the overflow drain now.

Seems like I have a leak somewhere else. But perhaps I should change this frost valve first? What would you recommend?

Sorted. It was the frost valve.

Thanks for all your help.

check the usual, but did you know lots of taps have micropore filters and yes its disgusting cleaning them, I drain and fill my fw system every trip, and remove the tap filters as well it would make you sick if you saw what I see,