I suggest logging onto the Texas tourist web site and request a free visitor package that includes a very good state map and fairly comprehensive travel book giving lots of info for the various areas of Texas. Depends on your interest as to whether you prefer one or more of the highly varied areas such as coastal plains, piney woods, blackland prairies, high plains, mountains, deserts, hill country, etc. Each has its own features and attractions. And many RV parks to accommodate you.
It is around 800 miles across the state east to west and 800 miles north to south.