So you have tank with liquid manure in it. The truck catches fire. Heat builds up. The plastic tank starts to burn. I would say that a vapor cloud would form even if it did not already exist.
What you speak of there is akin to a BLEVE. BUT that would only be the case if the tank stored methane. The tank in a stockyard would be designed to vent gas, not build up high pressure.
Full disclosure, years ago I was a Georgia certified firefighter. Trust me on this. Not trying to bust you, just know that what you wrote there is unlikely. Truly.