You could always give the old insurance company a ring and see what they say about you plugging that in overnight lol.
Or do a risk assessment and decide for myself.
He is right though it’s a modification and if not declared would void the insurance if it caught fire , Ime in the Fireservice and we do investigate fires in vehicles if it looks at all suspicious and a full FI report is done the insurance company can and very often do request a copy and then will not pay out , we get this all the time with boy racers modifying there cars and not declaring them just by adding alloy wheels and not telling them will result in no pay out . If you fit an immersion heater into a tank in a van you should have a qualified electrician check it and issue a certificate stating it’s fit for purpose and safe , without this again the insurance are going to use it as a loop hole not to pay out especially if it looks like that was the cause of the fire . On a side point years ago I was looking at putting an Emerson element in my van I spoke to a friend who is a qualified electrician and he said he wouldn’t fit it and advised against it stating they aren’t designed to be fitted in vehicles and plastic tanks . I think if ones want warm water it’s a very cheap and easy fit just make sure it’s 100% fitted right with the correct cable and socket in the house and preferably no bigger than 2 kw .
Spot on. I've fitted four immersions to four tanks - well five if you count the one I turned on with no water in and it went "ping". I know what I am doing. (except when I forget to put water in the tank! 
)
Let's take it a step further. You put an electric reel in your van - the electrics go bang and start a fire - what's the difference? What's the likelihood? Or your leisure battery (which isn't specifically designed for window cleaning application and nor is an aftermarket relay charging from your alternator) going bang?
There is a big difference between an electric reel taking 8 amps out of a battery to an Emerson heater taking 2-3 kw from the mains . Our vans , systems , reels wabasto heaters are all declared with the insurance company , they are also wired up correctly through a fuse board with the correct amp fuses fitted by a qualified electrician with supporting documentation that the insurance company have got copies of so should the worst happen Ime confidant there won’t be a problem 😂😂hopefully 😂😂😂
Immersion not Emerson.
A fire is a fire. Whether caused by 8 amps and 12 volts or 2000w and 240 volts. (Also 8 amps)
Have you declared your pumps on board and the power they draw? If so then great. The only thing I have declared is the DIY frame I have and the 650L tank. Because that might shift in an accident.
If I set fire to my van on the drive (by putting an oil filled radiator in the back in winter, by putting a fan heater in it on thermostat setting, putting a paraffin greenhouse heater in the back then that's my lookout. )
I put in a claim for a six month old car when an employee put what he thought was an extinguished barbecue near the front bumper in his garage. He came down on the morning and found the car burnt out completely. Insurance paid up as sweet as a nut.
I assess the risk of a burnt out van - from a faulty battery - wrongly wired pumps - an immersion in the back as phenomenally low. If it happens I'll take the hit. Ten years and five tanks later I have had no problem.
Because the job was done correctly.
