This depends on the construction and accesories of the inside of the camper. Here are some considerations.
1. Floor coverings:
2. Padding and any window coverings. Drapes and foam pads and pillows can absorb moisture and cause mildew smell to hang around. Replacing these materials will help renew and refresh your camper.
If the mildew or musty smell is within the walls of the camper, there will not be anything you can do about it.
Insects: Insects can be the source of the musty smell. You can look for insects in all the nooks and cranies of the camper. You may want to spray along the base boards and cabinet bottoms to make sure there are none.