They may be Nitrogen filled, but regardless, 80 PSI cold is where they should be regardless of air or Nitrogen. I wouldn't worry about filling with air or mixing the two as it's more important to have proper pressure than having Nitrogen. Tiny, tiny advantage to Nitrogen from what I understand. Minute advantage in temperature and condensation issues. Very difficult to find Nitrogen on the road, especially when you need it, plus good luck getting your 44' trailer into a tire shop that sells it, to top your tires. Just my opinion .
2015 Black F-350 Platinum Diesel Dually Crew Cab
2019 Riverstone Legacy 38MB (Full Paint/All Options)
2015 Voltage 3990 (Full Paint, All Options) Sold 6/18
2005 Harley Ultra Classic w/too many miles