It's not as easy as filling up a truck with gas or diesel, but after I've filled my trailer (with rocks, by hand) it gets tremendous fuel economy. The truck pulls it using very little extra fossil fuel too. What's more, unlike the material in my truck, the rocks don't have to be lifted off one-by-one from my trailer. They have a kind of vicarious latent potential energy. They can just be dumped using no human energy whatsoever and very little 'fuel energy' except 20 seconds of battery power.
I can deliver enough rocks in one dump load to keep busy building dry stone walls for nearly a day. When I return at the end of the day and the trailer is empty, I don't shake my head and get annoyed that it's 'empty' again. I think to myself 'good there's room for a lot more useful fossil rock material tomorrow'. The important thing is to keep filling the trailer.