adding to RoyB's comments.....not only do you need to have the fuse plugged in in the fuse box, but you need to have the wire hooked up...I recently found out that after 6 years of towing, that neither of my rigs were charged while towing because the 12VDC wire was tied off in the cable run...and when my RV mechanic pulled it out, and hooked it up, that I finally got 12v at my 2 trailer outlets.
He told me that a lot of the P/Us are like that, regardless of manufacturer. Dealer never told me that, nor did the owners manual.