I'd imagine a shop who sold it you would wriggle out of it somehow any way if the van happen to set on fire ie should have been installed by a gas man etc so you'd have to factor in cost of fitting by a gas man too!!!
To be honest if I was to find that a hot water system was a benefit to my business then that's the only way I would go.
A system bought a fitted by a qualified gas engineer.
Nothing to do with not being able to fit one myself but keeping myself right legally.
Iv witnessed a gas bottle go up on the back of a work lorry, it belonged to the firm I used to work for.
Luckily no one was injured but the lorry was ruined and a nearby property was damaged.
The insurance assessors went over the matter with a fine tooth comb and luckily for the owner everything was above board.