Alan,
If anything, a plumbers gas bottle would potentially be more hazardous than yours - they need to lift their gas bottle in and out of the vehicle for every job which could create a potential hazard due to correct and secure positioning of the bottle after use....there is more chance of a plumber just
'throwing in' his gas bottle at the end of the day.
Your gas bottle will remain securely positioned in the vehicle (in accordance with manufacturers installations) throughout its life - it does not need to be lifted in and out of the vehicle at each job site. If using 'bottle gas' you will only lift out and replace the empty bottle for the full bottle back into the secure position in your vehicle. if using Gaslow, you will
NEVER touch the gas bottle for any reason. Gaslow is EC approved for mobile vehicles.
I think the question your are asking is - can I have a bottle of gas 'plopped' in the back of my van for hot water....
Obviously, the answer is
NO. You have not adequately assesed all of the potential hazards/risks.
Your insurance company will no doubt assess the likelyhood of accident/hazard/risk based on your installation /equipment used

The mobile (and fitted Gaslow) gas bottles themselves are rated for 'mobile use' (within vehicles) so are unlikely to rupture in an accident.
A gas bottle itself (if securely fitted within the vehicle and filled in line with manufacturers instructions) should not cause any extra likelyhood of causing an accident/explosion.
You may be getting confused with the extra requirements for carrying
HAZCHEM....thats a whole new subject! and reserved for the likes of bottle gas distributors/delivery vehicles and petrol tankers

Youe need to fit a 'flammable gas' sticker in case your vehicle is involved in an accident/fire to make the fire (emergency services) aware you are carrying a gas bottle.
The gas bottle has more chance of injuring/killing you due to its incorrect placement (and flying through the air and hitting you!) than it ever does through explosion/fire hazard.
Conclusion - make sure its well fastend down with approved straps/anchors.