Can you at least get a good density figure from your supplier?
Are you using kWh as a general energy figure, or are you actually looking at how much electricity you could generate from a liter of LPG? (I will assume general energy unit).
The density is needed because LPG varies. In the US it is at least 95% propane, in some parts of the world it is a mixture of propane and butane, around 50%/50%. The energy per kilogram is nearly the same, but considerable variation per liter.
For US, a density of 0.508 kg/L is a pretty good figure (butane is 0.585 and a mix somewhere between.) Again for US conditions, energy density of 50.15 MJ/kg (HHV) or 25.48 MJ/L is a typical figure.
(Butane is 49.21 MJ/kg, not much different, but 28.79 MJ/L, quite different, due to density. A mix would lie somewhere between.)
My source, US DoE, uses MJ for energy and I used their figures. 1 kWh = 3.6 MJ. Just divide above by 3.6 if you want to get back to kWh.
Should you be generating electricity, you'll be lucky to hit 33% efficiency, so electrical output would be around 1/3 the above figures or lower.