Yes you need 50A service for a second A/C. As long as outside temps stay below 90, the single 15k should suffice. Any hotter than that, that size trailer needs a 2nd A/C. Unfortunately it is much cheaper to order from the factory than having it installed after the fact. I would recommend calling at least 3 different dealers to see what they would charge.
2005 Ford Excursion 6.8L, 3.73
2016 Eagle 324BHTS, P3, PP 3P