Found out after we bought one that our coach has surge protection built in but... I still prefer that the 300.00 or less unit we plug into the tower, post, outlet or whatever to take the hit instead or at least first. It saved us grief a few times. In Missouri the campground had both 50 amp circuits going to one leg. The unit kept everything from frying on that leg and half the coach was dark.
I'm not sure that was the diagonsis. It has been a long time. In Michigan a campground we visit a lot on a packed holiday weekend a surge then brown out was caught by the unit. Another occasion lightning hit power distribution, something or other and popped the unit. During a storm some trees and high tension lines, came down and all kinds of fireworks popped the unit.
Our unit is undamaged but I have read here and elsewhere, over the years, of RVs being saved by the units which themselves were sometimes fried. In some reports the mfg replaced the damaged units but not always. I don't remember the two or three brands or even what mine is at the moment. It plugs into the pedestal and cord into it.
There are post here on this forum and others of some RVers losing microwaves, air conditioners,tvs and every thing else from a surge,...and brown outs from low voltage. We don't have one of the EMS that boost voltage but it's on the list. We see low voltage we turn on the generator and disconnect for time being.