Can someone else chime in on the appeal process itself?
Secondly, you likely don't have a winnable appeal. After all, the JP would either need to have made an error of law or a serious procedural error. Findings of fact are given tremendous deference (i.e. the appeal court doesn't normally go against a finding of fact unless it is 'patently unreasonable'). They figure that the trial JP was in a better position to see witness expressions, body language, etc. Appeals are not re-trials. Rather, they are simply a review of whether the trial decision was fairly decided.
So, when you apply for an appeal, you are asking for 'leave to appeal'----meaning, the appeal court reviews your application to even see whether they will even devote any hearing time to your case. If they think there are no grounds for an appeal, your application for 'leave' will be refused. They don't have to provide any reasons why they denied your application for leave. Now, if they DO think your case has something of merit, then they will grant your application for leave. That just means you now will get a court date to argue your appeal. You then get a chance to make your arguments at your appeal hearing and they will decide. They may still decide that your case was fairly decided and deny your appeal (but at least you had an appeal hearing). In terms of decisions, the appeal court can send the case back for a new trial (i.e. the entire process starts all over again!), they can insert their own verdict (thereby avoiding another trial), or they can reject the appeal (in which case the trial decision stands).
With regards to your case, if both of you were inside the truck looking forward, its going to be hard to argue that you were keeping a watchful eye on whether the load truly was secure or not. Given that there was no tailgate on the truck, it is possible something (like the 2x4) slid----the officer (or anyone viewing from behind) would be in a better position to see that.