Your actual problem here is that you’re playing in single player. The slider which increases the length of respawn works well, and already does what you’re asking.
In single player, your PC is the server. To save on resources, it only renders the part of the map that you are interacting with. So if you kill everything in a camp, then leave render distance (even for 30 seconds), the camp will be completely respawned as soon as the game has to render it again. If however, you have the respawn set really slow, you can kill everything and stand in the camp for ages without them respawning. I’m guessing that you are filling your bags, leaving to return to your base and when you get back to the camp, you find it’s already respawned.
The solution to this is to play on a server. A server keeps the entire map populated, whether you are there or not. If you still wish to play single player, this can easily be done by running a (free) dedicated server on your gaming PC (or better yet, an old laptop), then transferring your single player saved game to that server. You can of course keep it password protected so no one else joins your server. For all intents and purposes, it will still be a single player game.
Happy hunting! 