LPG Alarm

Do you mean an alarm to detect a gas leak or a carbon monoxide detector?

I've never heard of anyone fitting the former. Assuming you have gas cylinders, the gas locker will be vented in the floor and, as LPG is heavier than air, any leakage will fall away. The same obviously applies to an underslung tank. 

If you mean a CO detector, your van should have one already fitted. Many seem to use Fire Angel.

Very common on boats, so readily available from Chandlers. I had a Mini Pilot Alarm fitted on my boat which worked fine. Be aware there is some battery drain to consider, but nothing a decent solar panel can’t deal with. Gas can leak from any pipe/fitting, not just the gas locker.

Presumably in a boat though it  can't escape through vents in the floor. 

Yep, that's why they're needed in boats😀. 

I'll amend what I said before - I’ve never heard of anyone fitting one in a caravan or MH😁

Yes, very clever, but same applies to leaks in MH in locations other than gas locker!  Perhaps you should inform Dellers he’s misguided for suggesting such an idea in your expert opinion?