Chargers have a ton of resources allocated to Tackle already, they will more than likely address secondary or DL... They have huge issues there.
It is very questionable that you can make the argument that you draft Tunsil at 3 with the resources they have allocated at left and right tackle. I guess you could move someone inside, but the truth is they have some pretty well paid guards too.
The only reason I think you can make the argument that Dallas could take him is that Free is aged and this would give us the opportunity to create a royal flush. It would help protect Romo for the next 4-5 years, and it would help our running game, and in helping our offense maintain possession of the ball, we'll have helped the defense.
I think we all assume law of diminishing returns, but we have to be careful. The offensive line is not wide receiver, nor is it running back. This isn't about how many touches of the football someone can get. It's about how well we protect and how well we run block and what you have to do with your skill position players in order to help your offensive line pick up the blitz.
If you can count on our five guys consistently taking their five guys, now even the 5th man blitz isn't wise. We no longer need to keep a tight end in to help block that 5th man. The defense now needs to send a 6th man in order to try and get consistent pressure. This means you only have 5 players in the secondary, with almost certainly one or two deep. That's 5 on 5 with a lot of holes in the defense.
An offensive line like we have right now can dominant the game. An offensive line we could have with Tunsil could revolutionize the game.