The reason to use nitrogen that was stated to me: the molecules are larger resulting in slower loss of pressure.
I've purchased tires from Costco for years - I always pay quite a bit less than the prices at tire stores, and they fill 'em with nitrogen. Very rarely do I need to add air.
Would I demand or pay to have them filled with nitrogen over air if it was an option? No.
To the OP - check your weights, inflate your tires at or just above the recommended pressures for your weights, and enjoy your trip! I run 65 lbs in the rear duals and 60 in the fronts, with plenty of capacity and a comfortable ride.