Yeah but, and? That's like saying you don't have a cigarette lighter/energy jack, or USB port, to power some electronic device because it uses extra energy.
Those do. In a car with a gas engine it might not be as noticeable, but the more drag you put on the alternator by powering more
"things", the more your gas mileage is reduced.
Or saying that you don't use an emergency gas generator because it uses gas. The purpose is as either an emergency power or as recreational power later or whatever you want to use it for without needing to run the car or some noisy generator.
But it would be more efficient to use a generator for recreational power later. Basically you are taking 100% of the energy from the EV's battery, losing some to friction in the motor, friction of the truck tires on the road, friction of the trailer tires on the road, friction of the generator motor in the trailer, then the transfer loss of electrons in the batteries in the trailer. So you end up with what 10% maybe 20% of what electricity you initially stored into the EV's battery??
A much better solution would be to use the EV to directly power your campsite. But, either way you are dramatically reducing the range of the vehicle.