There have been a number of threads on the forum about the benefits/non-benefits of using nitrogen in RV tires. I can't remember any thread that had any conclusive evidence that actually demonstrated any advantage that was cost effective or that substantially improved performance, safety or durability of the tires. ST tires are essentially "low tech" tires. I could see using nitrogen in "high performance aircraft", "dragsters" (where explosions could occur), and in "racing environments" where consistent high speed applications would be expected. In a 65 MPH trailer application, I can't see where the "extra 20% nitrogen" would be beneficial. Of course, if you really want to, who's to say you shouldn't spend your money any way you like......
Trailer Life did an article on RV tires some time back. Here is their direct quote on the subject:
"Nitrogen Inflation Green tire-valve caps denote the tires were inflated with nitrogen instead of atmospheric air. Nitrogen has some limited advantages over air, although no tire manufacturer requires the use of nitrogen. Atmospheric air contains about 79 percent nitrogen, and it’s impractical to inflate tires with 100 percent pure nitrogen, because the tire contains some air even when deflated. Therefore, if your vehicle’s tires have nitrogen in them, leave it in. Never run with lower pressure than needed due to an unavailability of nitrogen; use air to maintain pressure, if that’s what is available."
You can access the entire article here:
http://www.trailerlife.com/trailer-h.../rv-tires-101/