What you get is determined by the type of camping you do, where you go, how much storage space you have in your camper, and what you can afford and willing to pay for.
Sewer hose. My camper is 31 feet long. The black and gray drain is in the middle. That means it's approximately 15 feet from the drain to the back or the front of the camper. At a minimum, I always carried enough hose to reach the back or the front of the camper, in the event a full hook-up site does not have the sewer connection 10 feet or less from where I park to where it's located in the ground. Moving the camper 5 feet can make all the difference in the world. I carry 50 feet of hose. 2 twenty feet connected together and 1 10 feet to use at dump stations.
Water hookups can be equally as Questionable. So I always carry 200 feet of hose in 50 foot lengths (and I do not use the white one, just normal garden hoses that are, always have been, and always will be 100% dedicated to the camper for fresh water.
Electric Cord (30 amp) RV cord, I carry 3 extra 25 foot sections, giving me a total of 105 feet of potential extension cord (with the cord in my camper).
And yes, I have on a few occasions used every bit of all of this.
Very few Indiana State Parks have water spigots 25 feet from the connection of my camper. Most are a shared spigot, so no one can attach permanently to one. But I do plan my campsites within 200 feet of the spigot so I can attach the hose, fill the fresh tank on my camper and then unhook the hose so others can use the spigot.
I use a simple PUR water filter at the kitchen sink faucet.
I use a simple in-line water pressure regulator that attaches to the garden hoses.
I do not use any kind of surge protector (I probably should) but never have and have never had any problems. If anything, I've experienced LOW voltage at campgrounds, never high or over voltage. And theres really nothing you can do when the voltage is low.
The best think you can do is a driveway camp (camp out in your driveway for a week-end) withouth the assistance of your house. That will teach you real fast what you need and how much you'll need.
There's no right or wrong answer, every person is different, their camping needs are different, their financial resources are different. You just REALLY have to figure out what will work for you, and go from there.