Well a typically single axel caravan normally only has a payload of 150kg when empty regardless of size. That would take it up to it’s maximum weight per its chassis plate which it’s legal maximum weight. I think mines only 125kg.
So yes, you could put 150kg of cement in if you really want.
I’m not sure how that differs from the stuff you’d usually take when caravanning like gas bottles, food clothing, awning etc. You also have to account for any fitted accessories like a motor mover which is ~30kg, oh and a battery.
Fair enough if you are being pedantic.
Replace the caravan with trailer. If the trailer is empty versus full of cement, which one will have the biggest impact on EV efficiency?