100%. ONE HUNDRED PERCENT.

I guess it depends on the company, but pretty good I'd say.

I'd say pretty good. You were doing drugs on the job and in a company vehicle. Their insurance company would not be happy if they allowed you to stay. Driving under the influence, whether it be drugs or alcohol, is serious charge. An accident in a company vehicle could cost them hundreds of thousands of dollars.

A lot of companies have rules that forbid the use of drugs or other substances that distort your behavior, and you will be fired if they have those rules. An employer has the right to fire you for most any reason, provided it was not for your race or something like that.

Pretty much a certainty that you'll be fired.  Business that have vehicles need insurance.  Insurance is a lot cheaper for businesses that have zero tolerance drug policies.  Zero tolerance drug policies mean that employees who get caught with weed on company property, and the van is company property, get fired.  

Chances are high you'll be fired.

Possession of one ounce or less is a misdemeanor in Georgia.  If driving a van is part of your job and you were smoking dope while doing that, you most certainly will be fired.  That's a huge liability issue for your company. 

Maybe you will just get a written warning. But your situation is self inflicted. Hopefully you will learn from your mistake.