You do not NEED a surge protector. Some people like to have them as they offer protection against voltage spikes on the grid that could damage sensitive equipment in the trailer. Similar to how most of us have surge protectors on our computers and sensitive electronics in our home.
I have never had a surge protector on my trailer and have never had a problem. However, I am thinking of adding one next spring to be on the safe side.
So, not necessary but a level of insurance if you feel you want it.
DAN