Firstly, why don't you ask your insurance company - they will advise if you are or are not insured.
Depending on the vehicle, size of tank, shape of tank and where you want it in the back will depend if strapping is good enough or not.
For example - 1000 litre upright perched over the rear wheels would not be good strapped ( or bolted really )
A 400 upright against the bulkhead could be strapped safely.
Layflats upto 500 are OK strapped in
Darran