Fresh water being dumped - how can I stop it?

The A-S website still shows the Nuevo as having the Truma Combi.  The OP's symptoms perfectly match the Truma frost dump valve being open which it will be if the internal temp has dropped to 3 degrees or lower.  It can only be reset by pushing the button back in when the internal temperature has risen to 7 degrees.

Helen Robertson wrote:Thank you for your help all - resolved, once I found where the thing was housed 

It was the boiler drain switch that had tripped as most of you suggested- cant understand why this is not in the manual online

Cheers x

A-S put little effort into their manuals.  The current manual states on page 7-1 "If the vehicle is not being used during freezing conditions the water should be drained.  If the vehicle is being used in such conditions, refer to section 9 for details of the automatic draining of the heater and use of the water tank heaters."  If you go to section 9 it only repeats "If weather conditions are below zero centigrade and the motor home is not being used for habitation the water heater and water system should be drained." and it describes an older water tank heater that isn't fitted to the EC700 models.  However, if you look at section 9 of the older 2018 handbook (and why should you if you have a 2022 model!)  the frost control safety/drain valve is explained on page 9-16 as it is in the Truma Combi(E) Operating instructions on page 18.  The tank heaters fitted to EC700 models are only explained in the Harmony UMS User Instructional Handbook.

The frost control safety/drain valve should have been covered by the dealer on handover but I can well believe it wasn't.  This is an example of why this Forum is invaluable.