Because anything which could be said here as an "answer" is a guess, here's what I got for you. It would be my best guess there's an issue somewhere in the valve train and its wearing out. Most likely something is happening with the lifters and/or camshaft. It might also be there's an issue with the rocker arms, but less likely. My reasoning behind this "guess" is because there are really few parts in the LR4 engine (or any LSx style engine) where magnetizable shavings could be coming from.
There are two things here:
- Until the engine is opened up and examined, you won't know what's going on. Nobody can really tell you "this is the problem" for sure until it is opened up. Until then, it's a best guess.
- The engine would have been broke-in longggggg ago. Whatever is causing the metal shavings is a new event and new wear which should not be happening. Any engine which has any type of metal shavings in the oil (as you describe) is not in very good shape. It could die at any time and will only get worse with time. This engine is going out and there's no bandaid which is going to stop it. Only opening it up for inspection and replacement of the affected parts is going to solve its issues.
Good thing here is, if it is valve train related, you should be able to have the engine gone through with new parts for relatively cheap. An even better proposition here would be to get a replacement engine from a junkyard (or off of eBay or CraigsList). GM made a TON of the 4.8L engines for their trucks (they made even more of the 5.3L ones), which makes them fairly easy to find. It also makes them cheap. Under normal circumstances, the LSx engines can last for 250-350k miles without an issue with proper maintenance. I'd definitely suggest your circumstances aren't normal and it does happen.
Hopefully to make you feel a little better about the whole thing, from what you describe in your towing and maintenance, I doubt there's anything you've done which directly cause the problem or which you could have done better to prevent the issue in the first place. More than likely, whatever is going on has been happening for a while, just now it's happening at a lot faster pace. Something is misaligned enough where major wear is happening at an accelerated pace. You'll need to get this fixed sooner rather than later, as it isn't going to get better on its own.