I know some of the Redwoods have a safe built in right from the factory. Whether or not it would be beneficial to try and put one in your rig at this point is questionable in my opinion. It doesn't matter where you would install it, as cheaply made as most RV'S are, it wouldn't take but a minute or two to have it knocked out of there if somebody wanted it.
If you are really intent on doing it, I would drill holes in the bottom and bolt it thru the floor under the bed. I would try and hit one or two of the metal supports they have on the ceiling in the outside storage bay on a fifth wheel.
Personally, I would vote for a money belt, and leave the expensive jewelry at home.