If you're asking about a handgun the answer is NO. Some states/cities do not allow you to possess a handgun without official permission. Even though you may be passing through the state without stopping and FOPA (federal law) allows you to do so, some jurisdictions have a history of ignoring FOPA and making the owner jump through expensive legal hoops to make the charges go away and het his handgun back. Places like Chicago, NYS/NYC come to mind.
If you're in an accident and the police feel the need to impound your RV, will they find the handgun while taking inventory? It's a lot like playing the lottery...just how lucky do you feel?
Rifles and shotguns are perceived quite differently by police, especially if they don't have any black plastic!