Mine does the same thing if the inline fuse is blown. Are you sure the inline fuse on the tongue is still good? On my trailer, the inline fuse up front is for the jack only, it does not isolate the battery in any way, and last time I popped that fuse, my jack did exactly as you describe, it would only work if it was plugged into the TV.
2014 Jay Flight 28 BHBE
2015 RAM 2500 6.4L HEMI, Tradesman 4x4, 3.73
Blue Ox SwayPro (BXW 1503)
Upgraded from an REI internal frame backpack and a Eureka 1/2 dome tent!