Winter usage

How can I use my Motorhome in frosty weather when the safety valve on the water pump automatically opens and then discharges the water?

Well, the dump valve isn't on the pump, as such, it's on the inlet to the hot water tank. It opens below circa 3c and dumps the water to prevent the tank from freezing damage.
As it's right beside the tank (Not familiar with Nuevo but similar on Warwick I think) then it's always warm in that area, certainly much more than 3c as its also inside the van.

If setting off, fill the water tank, close the dump valve and run the hot taps until they stop gurgling. Then put the water heater on and leave it on until its thermostat switches it off, you may hear the gas solenoid valve closing.

Then, for at least that day if nothing else is done it'll be warm enough in there to keep it closed.

You can also use one of these ….

https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjuoezKo7X9AhUrQkEAHbsBB08QFnoECBAQAQ&url=https%3A%2F%2Fwww.leisureshopdirect.com%2Fgas%2Fcaravan-gas-water-heater%2Ftruma-combi-4-6-l.p.-gas-and-electric-heater-spare-parts%2Ftruma-frost-control-element&usg=AOvVaw3_aQCt5mMYTfkQgL6y8E9q

Just to be clear, the dump valve will open if the temp falls below the expected value INSIDE the van. If will not open just because there's low temperatures outside.