I have a 2014 Jurgens LunaGazer and very happy Almost from day one we had the issue of when we were connected to mains water pressure seperate to the water tank filler over a week the water tanks fill up and start overflowing Has any one had this issue and how did they fix it? Regards

Bit of a long shot but.... Some on here have piped mains water via a valve or two to be able to fill their tanks without needing to put a hose in the filler. Perhaps your van has that feature from the factory and the valves are open?

What likely has occurred for you is pretty common. Some van builders rely on the seals in the pump to stop water backflowing into the water tank(s). Regularly these pump seals eventually fail against the town water pressure and the tanks fill from the town supply. The town water pipework joins into the van's pipework somewhere on the delivery side (pressure side) of the pump not the tank side.

Some options for you:
. Some folk just always fill the tanks and use the pump for the vans water.
. Or you could fit a None Return Valve on delivery side of the pump. That is what I have done.
. Or Renew the seals on the pump. I understand for some pumps the seals are replaceable.

I have had the same problem and it was the Non Return Valve, as 'watsea' says just retro fit a new one, yes on the delivery side of the pump and make sure the flow arrow is pointing in the correct direction, there is plenty of info on how to do it on YouTube and it took me about 15 minutes and costs around $40, they are available at most Caravan Parts stores, I got mine from a Jayco Dealership.

Good luck