Yes, the brake light on the camper works.Before you go splicing into anything does you brake light on the camper work? If the light works then the ground for the camper is good and I'd just leave it as is no need to spice into anything.
The pink wire is a very small gauge wire and won't be able to support grounding for everything. I would still ground out the battery to the chassis with a large gauge wire like 4 awg.
Is your camper powered by your truck battery or an independent house battery? If truck battery then makes sense why no tie into the pink wire as your truck battery will be connected to your chassis as well and that completes the circuit. Curious how those with a house battery system are completing the circuit for the built in systems; it must either be "grounding" the house system to chassis or connecting the house system to the pink negative. IMO the brake light should have a separate negative to make this all easier.
Point taken that the pink probably shouldn't be the sole negative connection to the chassis since if a big load chose that path for whatever reason then it could overheat (although I'm not sure if one ever would, curious to hear what others think there).