I really dont think you need a caravan. And you are much more likely to find others going that way once in Southern Mexico or Guatemala. You could then try to link up with them.
I have traveled all over Central America (not in my RV) and have seen many people going in their RV's. Most of the roads are fine, some better than Mexico, and where there are people, you will find what you need.
Just go. You really dont need a security blanket for that trip.