Fishermen down here use beer kegs for their outboard fuel, almost bomb proof stainless steel. You can get those proquip jerry cans at most auto parts and hardware places around the country. We still have an ex army for carrying BD in for the last 30 years and a couple of proquip, had to renew the rubber gasket in the spout of the army one a couple of times, but that's it.

I can't imagine why anyone would buy marine fuel tanks or beer barrels to fit on a caravan drawbar when all the holders are designed for jerrycans.
I'd think twice about using those items with a van or trailer even if I had them, as jerrycans are so safe and easy to use.

It's best if the 20 litre jerrycan holders are a good design with padding and hold the steel cans nice and snug with no movement. The locking bar should cover the filler and lock them tight.  No movement and padding means no wear or rub though on the jerrycans.
But this also means the 20 litre plastic fuel cans won't fit as they are slightly larger.

A while ago Repco had a special on Proquip 10 litre Jerrycans, at just on half the price of 20 litre cans, so I bought eight of them instead of four 20 litre ones.
Ten litre cans are so much easier to handle than the larger 20 litre cans, you just need the right secure space to store them, which I have along the inside of my trailer.

I also have four external 20 litre jerrycan holders, two on my Hilux and two on the trailer, which have bars over the top to lock them in and cover the fillers. Its not unknown for fuel to be syphoned out of Jerrycans while still locked in holders if the fillers are uncovered.

I didn't know about the five year life on plastic fuel cans, but taking note of the advice on Proquip's website, if I had a plastic marine tank I'd be checking to see if the five year life applies to plastic marine tanks as well.
Charlie