RV "Oscar" rivets, otherwise known as blind rivets, work well for light to medium weight items and are very strong.  I've used them for towel bars, TP holders, coat hooks, etc.  For medium - heavy items, secure to studs.  A nice thing about these rivets is that they have a rounded head and shouldn't damage wiring or penetrate piping.  Just drill a hole deep enough to go through the 1/8" luan plywood.  
Wood screws can be used for lightweight items.
You can drill a small dia. hole like 1/16" or less and push the drill through by hand, or a piece of wire or small nail to see if it hits anything.  A small dia. hole won't be very visible if you have to abandon it.  If you have a fiberglass sided unit, there may only be studs/framing in the 4 corners and nothing in between.  Interior studs can be found by pressing against the wall until the luan doesn't flex.  Then look for the tell-tale tiny staple holes running vertically.  You will also find a thin tape over the luan where adjoining sheets butt together.
In some cases, you can pull off a light switch, thermostat, receptacle, etc. to see what is inside a wall cavity.  Interior partition walls are typically 1 - 2 1/2" thick.  Look for plumbing vents on the roof and they will give you an idea where they will be inside an interior wall.
Or do what the factories do - use a couple of itty-bitty, teeny-weeny U-shape staples.  :R  I've found a lot of sharp pointed #8 x 1" screws rammed into walls and ceilings with little regard for what is behind them.
