Space is always at a premium in campers, and most RV cabinets are made of very thin wood, so you can't screw into them. 

When I needed to add a shelf to hold some light kitchen items, I designed some simple brackets which use command strips to hold up a wire shelf.

I wouldn't rely on these for anything heavy, but for a few light items, they work well for me. 

You could use all “A” variant clips, or use 2 “B” clips in the back and 2 “A” up front like I did. Both variants have a slight indent to snap the shelf into the bracket with a nice snug fit, so it won't pop out easily when you go over bumps.