Nitrogen in tires has been discussed multiple times on these forums over the years. After lots and lots of jokes and kidding around about it, the general consensus from members on this forum boil down t to this:
1) It's a waste of money
2) It serves no advantage in the "family car" ... race cars traveling at 200 mph (maybe yes), but not the family car (or RV).
3) Nitrogen is mostly nothing but air anyway.
4) It's a marketing gimmick to make you want something (you know, create a new "fad" and then everyone wants in. The only people really gaining are those who rack in the $$$ off the consumers back falling for anything just so they will feel they are part of the "in group")
5) If the tire is currently filled with "nitrogen" it's OK to fill with normal air.
But, IF you fill your tires with helium, you trailer will weigh less! :B