Whether you extend the slides before leveling or after leveling depends on the manufacturer. On our Eagle we can extend the slides before or after leveling. As I recall the Newmar is somewhat different but I am not sure. Hopefully one of the newmar owners will jump in and advise. IMHO the best policy is to follow your coach manufacturers instructions. Back yard engineers can get folks into trouble. ;D
If you manufacturer instructs to level first then take that as fact. If your manuifacturer says to level before dumping the air bags then that is fact. Anybody, besides the manufacturer, says differently are making an effort, knowingly or unknowingly, to cause you problems.