When did you buy the trailer? Most states have 48-72 hour "Right of Recission" laws. That equates to two - three business days where you may be able to get your money back. Although you should have checked the weight rating on the truck prior to your visiting the RV dealership, I would contact the dealership manager and explain (politely, at least at first) you feel that the misinformation the salesperson gave you puts you and your family at great risk as well as others on the road. There's always the tactic of threats, i.e. going to the AG's office, BBB, etc., but if you start out polite you might be surprised.