I suppose you could buy a second hand underslung lpg tank from a vehicle scrappy and bolt it under the van?
Once they are 10 years old they have to be pressure tested and a new certificate issued. Usually costs more than the second tank. If it fails then you not only lost your initial outlay, but you still have to pay for the certificate cost.
Correct me if i'm wrong, but I think the system needs to have an annual safety inspection by a competent person as well.
If you go onto Ebay, there is a chap on there who advertises S/H tanks, but notice his 'safety' clause that covers him if anything were to go wrong. Insurance issues can also become a nightmare.
Spruce