Water Heater

Don't you just love the way a forum discussion evolves 

Just leave it on like you do at home . If we were really concerned about the impact caravans have on the environment we wouldn't drag them around the country and Europe  massively increasing our individual  carbon footprint?  

Turn it on 20 mins before I need it , then turn off ....

I certainly would not worry about the very small heat loss from the insulated boiler, because it will be lost to the inside of the Caravan, which you will be trying to keep warm in cold weather or on hotter days the loss will be insignificant as the differential between the two temperatures will be even less (See:  Newton's law of cooling) and the "maths/physics of heating 1kg of water by 1 degree C that requires 4185 Joules of energy (supplied by electricity or gas)"  as pointed out by Cornersteady.

Leave it on.  

Have always left it switched on ,never given it a second thought ...

As Mr Scott would say: you cannae change the laws of physics

just out of interest, water takes the the second highest amount of energy for any subtance to heat it by 1C, While water needs over 4000 joules per Kg for every 1C, copper only needs 385. Not a lot of people know that...

It means higher heating costs to heat it v keeping more water in us and on the planet. 

On my van a Lunar Quasar the water temp is only controllable if you use gas, on electric it heats up to approx. 65deg and just keeps on going , we usually leave it on but I sometimes get a niggeling worry that it could be doing harm.

what model of heater do you have? Also I'm confused, if it heats up to 65C and stays at 65C it can't keep on going? It must switch itself off at 65C or it would get hotter?

If one wanted to be miserly one could replace the on/off switch with a timer and have hours of fun deciding when next the water would be needed hot to set the timer.

What Corners says/implies. IIRC on my Lunar Delta (previous caravan) the lower temp option on gas was so you could economise on gas if need be.