Most travel trailer tires that you buy on the rims that are already mounted come out of the factory unbalanced.
Unless you notice excessive wear on the tires or vibration of your travel trailer then you might want to have the travel trailer tires balanced.
I've never balanced any of my trailer tires including travel trailer tires and never had a problem with them.
The main reason you want to balance tires on a vehicle is due to the front part of the vehicle and steering which you can feel the vibration.
But on a trailer it's usually not needed to balance the tires when mounting them.
But you can have the travel trailer tires balanced if you want too as it won't hurt anything.