Shitload is the impolite version of a shed load.
Is the correct answer.
The origin is pretty obvious to me "wow, that's a shed load of biscuits" would be the comment as you drove past it, hence "a shed load" would then be adopted to mean a lot.
No chance, a shed load of biscuits would be sold as seconds
A shed load isn't quantifiably large or small so in the "shed load of beer" context it can't possibly come from the lorry derivation, it comes from the garden shed derivation
And how bigs a garden shed? About as big as this piece of string?
Neither a shed load from a lorry, or a garden shed, has any specific sizing, so that argument doesn't really hold water.
I think the load lost from a lorry can vary in volume (a few bricks to a dozen pallets), a lot more than a garden shed (6' x 4' to 12' x 12'?)