If your camper has the foam board with a layer of wallpaper manufactured on it straight from the factory, you can paint it.... BUT... do VERY light coats of paint at a time. It takes A LOT of coats too, but if you paint that wallpaper until it is moistened, it will bubble at the seems where the weight pulls down on the moistened paper.
I would recommend a very short nap roller so that it isn't too saturated with paint and you can roll very light coats of paint on. After completely drying, you should be able to repeat the light coats until the wallpaper is covered.
JUST be very careful not to get the wallpaper wet!
You can buy a primer like Kilz and roll some light coats of that on first, it will help to hide any pattern in the paper and keep any dye in the wallpaper from bleeding into the paint when you paint it a color.
I actually recommend painting it a color that you can buy a matching caulk in. Here is a site that sells colored caulk:
Those wall board pieces are usually put together with seam bands in between two panels so that the boards can flex while being towed, and can shrink and expand when the camper is not being climatized. Sometimes these bands don't hold paint really well, and you could put a flexible caulk over them that is the same color as the paint that you painted the wallpaper with if you need to.
Every camper is different, so you would really have to look at it before you put the first roll of paint on... because that is the point of no return!
If the wallpaper is attached good and in good shape, it should not be a problem. Something else to consider is that campers have many areas that are really difficult to reach to paint. You may have to do the whole unit with a brush, versus a roller.
You may want to look into certain additives you can mix with paint to help fight mildew, since campers/rv are prone to moisture and mildew, etc...