[Festus2]

Sorry to hear about your dad's problems with his Sprinter. I think that your assessment that the structure is now comprimised is probably correct. If it has been leaking since 2006, it may also have been leaking prior to your father buying it. It sounds like there is extensive rot in the ceiling and walls and that problem will not be solved simply by finding and patching the leaks you have now.
In addition to the comprised structure, I would also be concerned about mold. You may not be able to see it, but I would suspect that it is present and could present a health hazard.
I would guess that extensive and expensive repairs would be necessary to your dad's Sprinter to correct the damage caused by the prolonged leaking. Considering that the Sprinter is now 10 years old, it may not be worthwhile spending that amount of money to get it repaired. In its present condition, you won't get much for it if you were to trade it in or sell it but "getting rid of it" may be worth considering.
Have you taken it in to see how extensive the rotting is and how much it would cost you to get it fixed? Might be worthwhile to get someone to take a close look at it to assess the extent of the damage.