Safe ? Depends for what ?
If you buy real good safe, it gets bolted down from the inside of the safe. You have to open it to have access to the bolts. Doesn't need to be hidden.
(If you decide to bury it, make sure you are not cutting into plumbing lines, electrical lines or whatever.)
Extra keys ? keep an extra TT key in the truck and an extra truck key in the TT.
Papers,cash and Docs ? no safe...just a hiding spot.
A plastic zip lock bag inside the sewer hose. Remember it's there so you don't flush it away while dumping. That can sometimes increase your dumping fee.
There are a few 'dead space' areas behind panels that sometimes unscrew to gain access inside. Areas under the fridge, furnace,water heater or bath/shower area. (sometimes there is a plumbing access panel to be able to access the shower drain etc)
Build a fake bottom inside a closet, under a carpeted step where you unscrew the step and lift it up or use the end of an overhead cabinet to construct a fake end. Use velcro to hold the fake panel in place. Stain it to match as close as you can to the original wood.
or use a fake electrical outlet , usually just large enough to hold whatever you need to hide.
***Use "decoy money" , maybe $20 in small bills in a small box on a countertop where a thief can see it right away. People that commit burglaries want to get in quick and back out with any cash they can, as fast as they can. The decoy $20 covers that. They really don't want to start taking things apart in an attempt to find something.
They need to carry the tools to do that, which is normally in many states, an additional crime on top of the burglary.