Mobile homes generally come with walls that are made of vinyl panels. These panels are attached to the walls, and the seams in between them are covered with strips. If you would prefer your walls to look more like drywall, you must cover the seams with drywall compound to make the walls appear smooth. This is not difficult, but first you must fix the walls so the compound will adhere to the surface.
Sand the surface of the vinyl either by hand or with a power sander, using medium-grit paper. This roughs up the walls so the primer will adhere better.
Paint the vinyl walls with an oil-based highly bondable primer paint. You will want to open doors and windows for ventilation while doing this, and wear a ventilation mask if you can't get enough air into the room. Paint with even strokes along the length of the wall. Use a roller for the large portions and a brush along the trim. It is best if you put down a drop cloth to protect the floor.
Let the primer dry overnight.
Spread drywall compound, typically called mud, along the seams between the panels. Use the flat edge of a trowel to spread a thin layer along the seam and force it into the gap between the panels. You can apply drywall tape if you want, but this is not necessary since you are not covering screw heads the way you do with drywall. If you apply tape on the mud, spread another layer of mud over it to cover it. Let the mud dry overnight.
Sand the mud smooth and apply another layer the next day. Make this application wider than the last, and taper it down to the panel's surface. Let this coat of mud dry another day as well. You can then paint the walls as you see fit.