My mini truck pop up camper.. with dim tail lights

Normally when things are terribly dim the reason becomes more obvious once you take off the tail light covers.

In your case, I think you have not got a good ground on one filament, so the hot wire feeds to the light, then it cannot come back but it still has a path out and it goes to the other side where it likely comes back through a brake light filament that doesn't glow because it's thick, then another run light filament, and then finds it's way out on a ground.