Essential, no. You can use your RV, and you can hook up at any campground without any of them.
However, I have all three.
I rather like the idea of a power surge, or a brownout not damaging the wiring in my RV or any of the appliances, etc. so I bought Progressive Industries device to protect both power surges or brownout situations. This is by far the most expensive item, but I finally decided I'd rather be safe than sorry.
I also rather like the idea of not having my plumbing blown out by excessive water pressure. So I bought a water pressure regulator.
The 50 to 30 amp converter is the least needed item, but it is relatively cheap and would come in handy should the 30 amp socket not be usable. You can step down the 50 to your 30 amp needs.