Call up Panasonic, General Dynamics, Allied Signal or TRW. They all sell computer systems that install in war machines, built to take a pounding.
Seriously though, if you look to the yellow pages for your area, and call some police / fire radio installers , they can direct you to an installer of military grade (police use them) computers. These are f-ing expensive, but have been built to take a pounding. The right case will have things like full length card supports to keep them from popping out of the slots, power and data connectors using screw-down covers or screw-in bulkhead type fittings, etc.
Or just buy a tower PC from alienware dell, and bolt it to the floor in your RV... it should last a while.