As Kono and Charlie are collecting gas samples from 12 stations, they are supposed to be collecting "87 octane" the lowest octane unleaded gas sold in the US. During one stop, Charlie finishes pumping gas into the gas can and hangs up the spigot in the middle position on the pump, which in the next closeup shot is revealed to be "Plus" grade gas. Unleaded gas is sold as regular (87, what they were after), some version of the word "Plus" (89), and then usually a brand-specific name for high grade which is 91 octane. Charlie was not pumping from regular unleaded, which is what they wanted to collect.
He could have easily got out of the trailer by going to the back and either using the back door or the emergency exit window all trailers have.