Q: Van shaking and misfiring.

Hello, a P0300 is a general misfire code without pointing to any one cylinder. There should be more codes present indicating which cylinder(s) have a misfire condition. You are correct in stating that a misfire is a combustion concern. To have a misfire there has to be a lack of ignition or a lack of fuel. The basics need to be checked, fuel psi, fuel filter (should be replaced at max every 2 years), condition of the distributor cap and rotor, spark plug and wire condition. If all the basics are good, then it would be time to connect a scanner and see what is going on, and what the computer is seeing. You may need to enlist the aid of a mechanic to help narrow down the cause.