I’m thinking Martinez torched the biters. He has access to the truck without raising suspicion, and we had a glimpse of him talking with Daryl in the last episode about his family. Just a hunch... I think he’s going to turn on the Guv at a critical point.
My personal opinion is that Milton did it, but who knows, you could be right. It had to be someone with access to the gasoline, which must have been locked up.