The biggest benefit is having a whole system ready for work in the morning regardless of the outside temps. If the temps are too low you wont be working anyway but nothing in your van will freeze, water in tank stays very warm all day & hoses are lovely & supple all day too. You are killing two birds with one stone- van heater & warm water.
I can't emphasise enough though- do the electrics properly. My best mate is a sparky & he gave me a proper reality check when I melted 13amp plugs! It's not a big job for a sparky to do, two/three hours work maybe.
I would also use a mechanical flange, this is the proper & correct way to install the element into a tank. It also gives you the ability to blank the hole off in the future should you need to & the tank isn't left with a gaping hole rendering it useless!
Here's the flange-
http://www.toolstation.com/shop/p38614The element just simply screws into this like it would on a domestic hot water tank.