What number do I call or who do I email to ask about changing my sites that were assigned to me for a September camping trip? We are coming with two trailers and they are not close to each other; also, sites might not be long enough?

Rangers office or the main texas park number

Use the Texas parks app

I was only a visitor there.

I agree, that's your best bet.

https://tpwd.texas.gov/state-parks/blanco

I would agree call The Ranger's office

Call the rangers office.