Hello there,
Thanks for your replies. Well I'm not so sure because this is my first camper. Let me explain how I noticed the leak.
I wanted to test the water pump and the systems as a whole. I filled up the fresh water tank, ran the pump and opened the sink to see if water come out. The pressure was so low that very little water came out of the sink, but I heard a leaking noise outside the camper. Went to see that the water was comming out from a dump pipe underneath the truma combi 6. Did a little research and found out that this dump valve was for the frost protection, and what do you know, my valve was opened, probably they flushed the system before they sold the camper to me. I close the valve and the huge leaking stopped, now the boiler was getting filled properly, but the pressure from the sink is still very low, is this normal? The pressure in the bathroom is much higher, is it low in the kitchen for a reason to save water?
Anyway i filled the boiler, warmed up the water, and I had warm water! The next day, I noticed that there was a wet spot right below the same dump pipe, I looked and saw that it is dripping, very slowly but constantly. So even with the frost protection closed, there is a drip, i think the water form the boiler inside the truma combi 6 was emptied overnight from the dripping. So my assumption is frost protection valve is leaking, because the dripping comes from the same pipe as in the begging when the frost valve was open.