If it is only a light weight drink your expecting to use on the shelf, then I would use those small corner shelf kits you see in the DIY stores.
Before fitting make sure your screws do not protude more than 15mm when inserted into the support brachets, this avoids the screws piercing the outer skin of the wall and also any internal wall panels that are double skinned. For added security I use some general purpose glue on the attatchment supports to help the screws holding the shelf bracket up.
Make sure you are not inserting screws into the wall board where electricity might be. Don't mount the shelf screws directly in line with lights or switches. Also if the mounting wall is pert of a shower room, check where the water pipes may be sited.
Also by having your caravan set level using your spirit level before you start, you can then use the spirit level to get the shelf level. Don't rely on other fittings to be upright or level. Obviously for asthetics you may have to vary the state of level a little to avoid making the current fittings look worse. (going with the fllow or slant).
It is a simple job to do, but covering all the key points take longer to write on paper than to perform.
Take your time and measure twice cut once or drill, as the case may be.
Good luck with your mission.
Steve L.