FEMA identifies brick or concrete buildings as the safest forms of shelter after a nuclear attack. Ideally, the best shelter would have few to no windows and a basement for camping out.
Schools or offices usually meet these criteria. Mobile homes, however, are considered too fragile to offer enough protection.
If there aren't any sturdy buildings within 15 minutes of where you're standing, it's better to find some form of shelter than stay outside. If you discover that there's a safer building close by, wait at least an hour before attempting to move. By that time, the potential radiation exposure would likely have decreased by around 55%.