Well, first the positive should go to a breaker, then on to your converter for power distribution. That should be where that T is. If you have a test light, check for power there, both legs if it's a breaker. If power is there go check where it goes into the distribution/converter box. You should have found the problem by now if everything works when plugged in. I'm a fan of checking and cleaning connections too, amazing what a little corrosion will do. Ground side also, just cause it looks good doesn't mean it is good.
Now, your harness. That's a mess unfortunately. The four wire, if correct, is ground, stop/turn left and right, and running lights. The others? Dedicated ground and power for charging the battery, and maybe trailer brakes though many pop ups don't have brakes. I'm afraid you may have to start tracing it out unless someone can say for sure.
Good luck, keep up posted on your progress!
I just looked again, that four wire setup is just crudely added to the harness isn't it? Yeah, time to rewire that correctly.
Now-2014 Sierra 346RETS 5er BUB
Then-2002 Keystone Springdale 286RLDS TT
Nights camped in 2014-28, 2015-127, 2016-10