I have fitted immersion elements into tanks on a couple of vans I've had over the years it can work well
You could dangle it but get that wrong and at the very least you'll damage your tank, or at worst electrocute yourself.
When I fitted one I cut a hole in the side of the tank, obviously you need to be able reach inside to fit the washer etc and cut the hole very precisely so your tank is still water tight after you're done! A good grade silicon sealant inside and out and it won't leak. buy a good grade flex, and a good extension lead that can cope.
Advantages:
Cheap option
works ok
Warm water if left on overnight
the tank acts like a big radiator and keeps your van and equipment all toastie.
Disadvantages:
Get it wrong and you'll have major problems
water and electricity don't mix well
useless if you forget to put it on or there is an unexpected frost.
people will take the mick if you do things yourself instead of paying for someone else to do them (who cares)
additional condensation in the van
I haven't fitted one to my present van I live on the South coast and frost/snow is pretty rare here.
Hope that helps