I remember one time having this wonderful meeting with the five big bosses of the five different business lines on sorting out what goes into the top of the product backlog. They all had enough on their plate of needs to fill the whole development pipeline, and little interest to give up on their own needs. It was not a collaboration, but a negotiation and it wasn't going so well.
A particularly funny experience was when we tried giving them visual tokens of how much they could contribute to decisions, it was some form of a dot vote. Everyone voted their own, everything was equal. But then, the game balance was broken by giving one token to the person outside this decision group, and with one token she could tip the balance to choose whatever she wanted. And all of a sudden, the tone of discussions changed into trying to find more commonalities on what the needs of each business area were.
This seems to be a recurring problem for me. I hardly ever have a product owner who could actually make the decisions for the different stakeholders in a balanced way - the team ends up helping with that work and I find that it's great, adding value to that discussion. But I'm still on the lookout.
What is the best way you've found, when relying on a single source of truth is not available, to balance the needs of five major groups of stakeholders? My magic lies within making the batch size smaller and giving each a turn. We can do it all, but not just today. If you have to wait a week to get something of value, it seems to be much better than six months.