That's funny, I had the same problem last spring. It indeed was my power transfer box. There's a little LED on the board that, according to the manual, should indicate it's operation. The frustrating thing about mine was that it magically fixed itself.
In the mean time. What I did while mine was broken was I took the power cord from the charging unit (in the compartment with the shore cable) and plugged it in to a standard 120V extension cord. I plugged that 120V cord into the 120V power along with the 50AMP on the post at the campground. So when I was hooking up my electrical I just had to hook up 2 cables instead of one. Not really a big deal for me.
2000 Forest River Reflections 327s DP
Toad = 2009 VW Tiguan.
Coupla bikes, coupla dogs, coupla 30-somethings wishing they were retired.