So I carried a Sunset Sass crate from the NCR camp on the west of Camp Searchlight, down to Searchlight Airport.

To use as a step up onto the terminal building.

When I came back around, about a week later, it was gone.

So do these crates revert back to their original locations after a set period of time?

Would it matter if they were emptied or contained their original contents?

And does anyone use anything else for steps, so far I have used dead radscorpians and this crate, both of which disappeared.

Cells will reset to the stock items that are in the cell if you are gone long enough. It's part of the process to clean up the weapons that get left lying around after a battle. Not sure how the game differentiates between Statics or cell based items and something that you drop on the ground (which should be there until an NPC picks it up).

As for climbing on to things. Since you are not supposed to climb onto most buildings in this game, the roofs are very boring, even to the point of being undetailed or even finished to look like the rest of the building. It's where the devs get to be lazy. If I need to climb up to something, then I just use TCL and walk up there.