Who says you have to deflate your tires to 18 PSI "to get to the beach" ???
Second questions, why? At 18 psi you can easily damage your tires as they are still carrying the full load of your coach.
I'm guessing that when re inflating your tires it will make no difference which tire you do first.
For the record the air from your compressor is 78 percent nitrogen.
You will likely have more of a problem with the corrosion from the sand/salt encountered on the beach. Hopefully you will be able to thoroughly fresh water flush the undercarriage of your coach when this adventure concludes.
Good luck.
:S