The problem is that the immersion tends to heat the water immediately around it and also directly above it so you get a hotspot or hot quarter of the tank - setting for 30 degrees in say a 400L tank will have it clicking off when much of the water is unheated.
There are two ways round this - put your pump outlet back into the top of the tank so that there is some circulation - but you will need an old spare battery for this.
Or - set the immersion for 70 - 90 degrees so that when you drive off the whole lot will mix and give you about 40 degrees (luke warm to the touch).
Also only heat what you need so if you will only use about 200L then heat 250.
The water will stay warm for hours.
Welcome to the world of warm! (on the cheap)
Oh and working at 30 plus means you will more likely have hoses and pole hose and brush hose "popping" off if the connections are worn.