Misconduct, for unemployment purposes, is willful and knowing misbehavior as defined in the company handbook (if they said it is misconduct to do this and this and this, and you signed off on it, and then you violate one of these policies, that is misconduct) This type of misconduct is usually traced by a series of warnings and steps of progressive discipline: verbal, written, suspension, termination; as you are warned that your job may be in jeopardy, and given a chance to change the behavior and you refuse to do so.
If this is not the case, you must have committed gross misconduct, which is defined as misconduct so blatant, so severe that you knew it was wrong and that any reasonable individual would know it was wrong, even without a company policy stating that specifically or without prior warnings. Punching out your supervisor is usually an example of gross misconduct.
In the appeals hearing, you will need to be very specific about what training you had received, what you told them about your proficiency before the accident, when you were scheduled to be tested, what you were told when you were directed to take this vehicle out, what you were told when you were terminated. You are making the case that you did not have adequate training on this vehicle, that it was the responsibility of the company to provide you with the training, that it was the policy of the company to have only people who had been tested for proficiency on this vehicle use it, and that to have refused this assignment, though you had not been adequately trained would probably have meant you might have lost your job due to insubordination. You need to stress that the training you received from your supervisor the day before was cursory, you did not receive the hands-on on the road training that was required by the employers own policy.
Now, that said, I hope you did not do a hit and run, or a driving while impaired, or have a major accident deemed due to your own negligence. (Change lanes and run into a school bus, say.)If so, this kind of misconduct could be major misconduct, whether you had been trained in this vehicle or not. You know better than to do this, even without training. If the accident was due to your not knowing how to operate the brake, your inexperience at steering, inability to judge how much room you needed to turn, etc, this could be a lack of training issue.
So the determination of whether this claim will be approved or denied will rest much on some things that have already happened and cannot be changed. No one can tell you whether you will be approved or denied. You will state your case, then the employer will have a chance to state their case that they had good misconduct reason to terminate you.
Remember that misconduct by definition has implications that (1) you knew what the rules were (2) you had the ability to follow the rule or procedure, you had the skill, ability, and training to do the job adequately within the rules (3) that you willfully and deliberately did not follow the rule, procedure, or process by your own choice.
Be sure to keep filing your weekly certifications until the hearing. If you were to win, you will be back paid for each week for which you have filed.
The classic case in a similar situation is illustrated by the sneezing employee. Driving a company vehicle, an employee is seized by a fit of sneezing, and rear ends another vehicle. Is this considered misconduct, grounds for termination? Generally not. Unless he was sniffing cocaine.