If a person is risk adverse, and you plan on RVing for another 10 years or so, your retired, and you have the financial ability, I would upgrade now to a new or near new tow vehicle. That would carry you on warranty half way through your planned RV usage. Biting the bullet now, could provide an lot of peace of mind as years and miles click by.
If money is tight I would roll the 6.0 out the door and locate a used Duramax or Cummins.
I see no scenario where keeping and updating the 6.0 makes sense for this guy. Personally if I was unfortunate enough to own a 6.0 (while I have not, the family farm had several of them) I might roll the dice and fix it, but certainly not if I am retired, want something dependable and don't want to wrench on a POS on the side of the road. I don't see Bulletproofed 6.0's bringing any more money, at least not here. Any money put into a 6.0 is virtually wasted as to resale (be it repair or bulletproofing).
The last place you want to be is FORCED to drop big money into it. Right now, the only asskicking your taking is on the terrible resale, I would consider that a win (in the case of a 6.0). Keeping it, jeopardizes that fortunate position.