As a retired LEO, who spent time in a drug unit, I saw some really good places to hid money/drugs/jewelry. So ask yourself this, if the bad guys (drug dealers) use certain places to stash their valuables wouldn't they easily find yours in these same places?
Probably the worse place, especially in a RV, is a mounted safe IMHO. A safe is hard to hide once someone actually gets inside and it just screams "I have really valuable stuff to steal here!".
Most Keystone Montana fivers come with a secret hiding place. All Montana years/models are different. My 2008 had a really good one, my new unit is okay but not as good as my old unit.
For cash, or large amounts of jewelry, pick up one of these and mix them in with other stuff in your pantry http://www.misdefenseproducts.com/Hiding-Places-for-Valuables-p-1-c-272.html
You don't have to purchase from these folks, it is just a site that is out there. These come in handy also if you are leaving your RV to be worked on and you want to remove some cash or jewelry from the unit. Instead of taking the chance of someone seeing you remove the valuables, you could just remove some pantry food items in an old Walmart bag, with one of the cans being the "hidden safe" can. I have even used such a "hidden safe can" and just let it roll around on my rear floor board with some other junk. Now who would think that warm, possibly dented, old can of coke had $5,000.00 cash stuffed in it?
On larger items, such as firearms, there is no really good solution for keeping them 100% safe in an RV. Trigger locking and hiding them is about all you can do.
You have to be smarter than most of the scumbags, not just smarter than a few of them.