Hi I use a 2.3 kw wallpaper stripper element to heat 200lts of water and then add this to my van tank , the advantage of the stripper element is that it has a nut and washer to fix it in place.
My heater is on a timer and is on for about 4 hours ish and gets to roughly 40/50 ,so when mixed with van tank lovley warm water most of the day.