Good advice Bruce, ... but keep in mind, I'd bet most of the KLR's you met were the USA "A" models, which come standard with a 23L fuel tank. Far as I know, the "C" model is virtually unknown outside the EU. I've never even seen one. Far as I know, never imported into USA, pretty rare this side of the pond.

But as you say ... a plastic jug (or two) of just about any kind will suffice to carry an extra 8L or so of fuel.

Coastal Chile and Peru' and Bolivia's Uyuni salt flats, you for SURE will need more than 14L. Carry 8L extra at least. ... and never pass a fuel-up opportunity.

Alternate:
You may post around and find an "A" version tank for sale cheap. Will it fit on the "C" model? I've no idea.

I'm sure "A" tanks are for sale cheap in USA. But shipping one to EU would not be worth it. (probably $150 usd to ship?) But if you begin your S.A. ride in the USA, then it'd be easy to pick up a larger tank once in US. Just a thought.

Keep in mind, with a strong head wind (common) and a fully loaded KLR, your fuel economy may drop down far below your average. Also, sometimes only low octane fuel is available, this can reduce range further (kms per liter).

The other fuel issue common in S. America are labor strikes and political protests. This especially true in Bolivia, Peru', Ecuador and Colombia. Often times roads are blocked and fuel stations shut down or empty.

Also, at times, fuel stations RUN OUT of fuel because fuel trucks are blocked from making deliveries. Most travelers work around these obstacles. But pay attention so you don't get caught with no access to fuel.