Welcome to the forums and congrats on getting your Montana.
About manuals? Keystone has a very generic manual that is applicable to any camper. They are informative to a degree, but not specific to your model.
For everything else, if manuals are provided, you are fortunate. Otherwise, each item's manufacturer in your camper will have to be located separately.
You are doing the right thing by searching the internet and downloading the manuals you need. I have several I've downloaded. It is only those items I've added after market that I have paper copies of. (Those things I've purchased myself), and then, even some of them don't have manuals ... like my small Roku televisions or my Boise sound bar. So, search the "net", download, print what you need, and maintain your own library. That's what we all have to do.
About the back up camera. Furrion is probably the most known, only because Keystone and Keystone dealerships push them. There are other brands "out there" that may work better for you, are cheaper, and will serve you better than Furrion. But the back-up camera is an item the manufacturer does not provide, because personal preference is simply too varied. It's something like, the RV manufacturer supplying your camper with cookware or the blankets for your bed. Those items are supplied by the owner.
Here's a link to the Furrion site. Again, Furrion is not the cheapest, but it probably is the most popular: Click here.
One more thing you may add to your arsenal of information about your camper, is to search YouTube videos. Pretty much, every RV model ever created has a video on it. Pretty much every item ever manufactured has a video on it. In addition, pretty much ANY repair you need to do on your camper, there will be a video on it. Book mark these video sites and keep them for future reference as you stumble into them. These can be extremely valuable in your future.
I wish you well with your new Montana and hope you have great success making new memories as you travel. Happy Camping! And again .... Welcome!