So, might seem odd that I'm bumping such an old thread, but I wanted to share something, in case anyone else came here for solutions like I have.
So, I've been working my way through the P-51 Challenge campaign and I got stuck on mission 18; today I did some searching to see if anyone has devised a technique. Much to my dismay I find everyone saying you either skip this mission or use cheats. Like many, I was getting so close, but not quite doing it...
Well, I was determined and wanted to have a few more goes before I skipped it. Just now, I successfully hit the refueller, burning all but two of the trucks. Dropped the second bomb on another truck, leaving me with one last truck and no weapons... Guess I have nothing to lose if I kamikaze, right? Turns out, that wins the mission!
You have to score a direct hit on the truck - you can't just crash into the ground two inches away, you have to crash INTO the truck. But get it right, you will have the camera panning away from a big explosion with a voice saying; 'Well done, the vehicles have all been destroyed. You may end the mission now.' I have just ended the mission and am now starting mission 19. :)