You pay your price, and you takes your chances.
You don't *have to have* a surge protector, but if you travel to many campgrounds you'll soon see that electrical management at campgrounds is very inconsistent and sometimes very poor. With poor wiring, and dozens of campers plugging in and out, voltage management isn't the solid 120V you see in your sticks-n-bricks.
In 20+yrs of RVing I've always had at least the cheap $50 surge supressor, can't say if it's ever saved me or not. In the last ~7yrs I've had the more expensive ones that deal with undervoltage conditions (toughest on some appliances like AC), I can say that these have warned and/or locked us out of *several* problems.
For ~$100 you can get ones that warn you when you plug in, for ~$300 you get ones that monitor and lock out if voltage goes too low. We travel 4-6months per year, prefering state parks/municipal parks/national parks/COEs, and probably experience low voltage or floating neutral conditions at about 10% of hookups. Not regularly, but not unusually either.
